Related
I read an article about the getattr function, but I still can't understand what it's for.
The only thing I understand about getattr() is that getattr(li, "pop") is the same as calling li.pop.
When and how do I use this exactly? The book said something about using it to get a reference to a function whose name isn't known until runtime, but when and why would I use this?
Objects in Python can have attributes -- data attributes and functions to work with those (methods). Actually, every object has built-in attributes (try dir(None), dir(True), dir(...), dir(dir) in Python console).
For example you have an object person, that has several attributes: name, gender, etc.
You access these attributes (be it methods or data objects) usually writing: person.name, person.gender, person.the_method(), etc.
But what if you don't know the attribute's name at the time you write the program? For example you have attribute's name stored in a variable called attr_name.
if
attr_name = 'gender'
then, instead of writing
gender = person.gender
you can write
gender = getattr(person, attr_name)
Some practice:
Python 3.4.0 (default, Apr 11 2014, 13:05:11)
>>> class Person():
... name = 'Victor'
... def say(self, what):
... print(self.name, what)
...
>>> getattr(Person, 'name')
'Victor'
>>> attr_name = 'name'
>>> person = Person()
>>> getattr(person, attr_name)
'Victor'
>>> getattr(person, 'say')('Hello')
Victor Hello
getattr will raise AttributeError if attribute with the given name does not exist in the object:
>>> getattr(person, 'age')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Person' object has no attribute 'age'
But you can pass a default value as the third argument, which will be returned if such attribute does not exist:
>>> getattr(person, 'age', 0)
0
You can use getattr along with dir to iterate over all attribute names and get their values:
>>> dir(1000)
['__abs__', '__add__', ..., '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']
>>> obj = 1000
>>> for attr_name in dir(obj):
... attr_value = getattr(obj, attr_name)
... print(attr_name, attr_value, callable(attr_value))
...
__abs__ <method-wrapper '__abs__' of int object at 0x7f4e927c2f90> True
...
bit_length <built-in method bit_length of int object at 0x7f4e927c2f90> True
...
>>> getattr(1000, 'bit_length')()
10
A practical use for this would be to find all methods whose names start with test and call them.
Similar to getattr there is setattr which allows you to set an attribute of an object having its name:
>>> setattr(person, 'name', 'Andrew')
>>> person.name # accessing instance attribute
'Andrew'
>>> Person.name # accessing class attribute
'Victor'
>>>
getattr(object, 'x') is completely equivalent to object.x.
There are only two cases where getattr can be useful.
you can't write object.x, because you don't know in advance which attribute you want (it comes from a string). Very useful for meta-programming.
you want to provide a default value. object.y will raise an AttributeError if there's no y. But getattr(object, 'y', 5) will return 5.
For me, getattr is easiest to explain this way:
It allows you to call methods based on the contents of a string instead of typing the method name.
For example, you cannot do this:
obj = MyObject()
for x in ['foo', 'bar']:
obj.x()
because x is not of the type builtin, but str. However, you CAN do this:
obj = MyObject()
for x in ['foo', 'bar']:
getattr(obj, x)()
It allows you to dynamically connect with objects based on your input. I've found it useful when dealing with custom objects and modules.
A pretty common use case for getattr is mapping data to functions.
For instance, in a web framework like Django or Pylons, getattr makes it straightforward to map a web request's URL to the function that's going to handle it. If you look under the hood of Pylons's routing, for instance, you'll see that (by default, at least) it chops up a request's URL, like:
http://www.example.com/customers/list
into "customers" and "list". Then it searches for a controller class named CustomerController. Assuming it finds the class, it creates an instance of the class and then uses getattr to get its list method. It then calls that method, passing it the request as an argument.
Once you grasp this idea, it becomes really easy to extend the functionality of a web application: just add new methods to the controller classes, and then create links in your pages that use the appropriate URLs for those methods. All of this is made possible by getattr.
Here's a quick and dirty example of how a class could fire different versions of a save method depending on which operating system it's being executed on using getattr().
import os
class Log(object):
def __init__(self):
self.os = os.name
def __getattr__(self, name):
""" look for a 'save' attribute, or just
return whatever attribute was specified """
if name == 'save':
try:
# try to dynamically return a save
# method appropriate for the user's system
return getattr(self, self.os)
except:
# bail and try to return
# a default save method
return getattr(self, '_save')
else:
return getattr(self, name)
# each of these methods could have save logic specific to
# the system on which the script is executed
def posix(self): print 'saving on a posix machine'
def nt(self): print 'saving on an nt machine'
def os2(self): print 'saving on an os2 machine'
def ce(self): print 'saving on a ce machine'
def java(self): print 'saving on a java machine'
def riscos(self): print 'saving on a riscos machine'
def _save(self): print 'saving on an unknown operating system'
def which_os(self): print os.name
Now let's use this class in an example:
logger = Log()
# Now you can do one of two things:
save_func = logger.save
# and execute it, or pass it along
# somewhere else as 1st class:
save_func()
# or you can just call it directly:
logger.save()
# other attributes will hit the else
# statement and still work as expected
logger.which_os()
Other than all the amazing answers here, there is a way to use getattr to save copious lines of code and keeping it snug. This thought came following the dreadful representation of code that sometimes might be a necessity.
Scenario
Suppose your directory structure is as follows:
- superheroes.py
- properties.py
And, you have functions for getting information about Thor, Iron Man, Doctor Strange in superheroes.py. You very smartly write down the properties of all of them in properties.py in a compact dict and then access them.
properties.py
thor = {
'about': 'Asgardian god of thunder',
'weapon': 'Mjolnir',
'powers': ['invulnerability', 'keen senses', 'vortex breath'], # and many more
}
iron_man = {
'about': 'A wealthy American business magnate, playboy, and ingenious scientist',
'weapon': 'Armor',
'powers': ['intellect', 'armor suit', 'interface with wireless connections', 'money'],
}
doctor_strange = {
'about': ' primary protector of Earth against magical and mystical threats',
'weapon': 'Magic',
'powers': ['magic', 'intellect', 'martial arts'],
}
Now, let's say you want to return capabilities of each of them on demand in superheroes.py. So, there are functions like
from .properties import thor, iron_man, doctor_strange
def get_thor_weapon():
return thor['weapon']
def get_iron_man_bio():
return iron_man['about']
def get_thor_powers():
return thor['powers']
...and more functions returning different values based on the keys and superhero.
With the help of getattr, you could do something like:
from . import properties
def get_superhero_weapon(hero):
superhero = getattr(properties, hero)
return superhero['weapon']
def get_superhero_powers(hero):
superhero = getattr(properties, hero)
return superhero['powers']
You considerably reduced the number of lines of code, functions and repetition!
Oh and of course, if you have bad names like properties_of_thor for variables , they can be made and accessed by simply doing
def get_superhero_weapon(hero):
superhero = 'properties_of_{}'.format(hero)
all_properties = getattr(properties, superhero)
return all_properties['weapon']
NOTE: For this particular problem, there can be smarter ways to deal with the situation, but the idea is to give an insight about using getattr in right places to write cleaner code.
# getattr
class hithere():
def french(self):
print 'bonjour'
def english(self):
print 'hello'
def german(self):
print 'hallo'
def czech(self):
print 'ahoj'
def noidea(self):
print 'unknown language'
def dispatch(language):
try:
getattr(hithere(),language)()
except:
getattr(hithere(),'noidea')()
# note, do better error handling than this
dispatch('french')
dispatch('english')
dispatch('german')
dispatch('czech')
dispatch('spanish')
I sometimes use getattr(..) to lazily initialise attributes of secondary importance just before they are used in the code.
Compare the following:
class Graph(object):
def __init__(self):
self.n_calls_to_plot = 0
#...
#A lot of code here
#...
def plot(self):
self.n_calls_to_plot += 1
To this:
class Graph(object):
def plot(self):
self.n_calls_to_plot = 1 + getattr(self, "n_calls_to_plot", 0)
The advantage of the second way is that n_calls_to_plot only appears around the place in the code where it is used. This is good for readability, because (1) you can immediately see what value it starts with when reading how it's used, (2) it doesn't introduce a distraction into the __init__(..) method, which ideally should be about the conceptual state of the class, rather than some utility counter that is only used by one of the function's methods for technical reasons, such as optimisation, and has nothing to do with the meaning of the object.
Quite frequently when I am creating an XML file from data stored in a class I would frequently receive errors if the attribute didn't exist or was of type None. In this case, my issue wasn't not knowing what the attribute name was, as stated in your question, but rather was data ever stored in that attribute.
class Pet:
def __init__(self):
self.hair = None
self.color = None
If I used hasattr to do this, it would return True even if the attribute value was of type None and this would cause my ElementTree set command to fail.
hasattr(temp, 'hair')
>>True
If the attribute value was of type None, getattr would also return it which would cause my ElementTree set command to fail.
c = getattr(temp, 'hair')
type(c)
>> NoneType
I use the following method to take care of these cases now:
def getRealAttr(class_obj, class_attr, default = ''):
temp = getattr(class_obj, class_attr, default)
if temp is None:
temp = default
elif type(temp) != str:
temp = str(temp)
return temp
This is when and how I use getattr.
Another use of getattr() in implementing a switch statement in Python. It uses both reflection to get the case type.
import sys
class SwitchStatement(object):
""" a class to implement switch statement and a way to show how to use gettattr in Pythion"""
def case_1(self):
return "value for case_1"
def case_2(self):
return "value for case_2"
def case_3(self):
return "value for case_3"
def case_4(self):
return "value for case_4"
def case_value(self, case_type=1):
"""This is the main dispatchmethod, that uses gettattr"""
case_method = 'case_' + str(case_type)
# fetch the relevant method name
# Get the method from 'self'. Default to a lambda.
method = getattr(self, case_method, lambda: "Invalid case type")
# Call the method as we return it
return method()
def main(_):
switch = SwitchStatement()
print swtich.case_value(_)
if __name__ == '__main__':
main(int(sys.argv[1]))
setattr()
We use setattr to add an attribute to our class instance. We pass the class instance, the attribute name, and the value.
getattr()
With getattr we retrive these values
For example
Employee = type("Employee", (object,), dict())
employee = Employee()
# Set salary to 1000
setattr(employee,"salary", 1000 )
# Get the Salary
value = getattr(employee, "salary")
print(value)
I think this example is self explanatory. It runs the method of first parameter, whose name is given in the second parameter.
class MyClass:
def __init__(self):
pass
def MyMethod(self):
print("Method ran")
# Create an object
object = MyClass()
# Get all the methods of a class
method_list = [func for func in dir(MyClass) if callable(getattr(MyClass, func))]
# You can use any of the methods in method_list
# "MyMethod" is the one we want to use right now
# This is the same as running "object.MyMethod()"
getattr(object,'MyMethod')()
It is also clarifying from https://www.programiz.com/python-programming/methods/built-in/getattr
class Person:
age = 23
name = "Adam"
person = Person()
print('The age is:', getattr(person, "age"))
print('The age is:', person.age)
The age is: 23
The age is: 23
class Person:
age = 23
name = "Adam"
person = Person()
# when default value is provided
print('The sex is:', getattr(person, 'sex', 'Male'))
# when no default value is provided
print('The sex is:', getattr(person, 'sex'))
The sex is: Male
AttributeError: 'Person' object has no attribute 'sex'
I have tried in Python2.7.17
Some of the fellow folks already answered. However I have tried to call
getattr(obj, 'set_value') and this didn't execute the set_value method, So i changed to getattr(obj, 'set_value')() --> This helps to invoke the same.
Example Code:
Example 1:
class GETATT_VERIFY():
name = "siva"
def __init__(self):
print "Ok"
def set_value(self):
self.value = "myself"
print "oooh"
obj = GETATT_VERIFY()
print getattr(GETATT_VERIFY, 'name')
getattr(obj, 'set_value')()
print obj.value
I read an article about the getattr function, but I still can't understand what it's for.
The only thing I understand about getattr() is that getattr(li, "pop") is the same as calling li.pop.
When and how do I use this exactly? The book said something about using it to get a reference to a function whose name isn't known until runtime, but when and why would I use this?
Objects in Python can have attributes -- data attributes and functions to work with those (methods). Actually, every object has built-in attributes (try dir(None), dir(True), dir(...), dir(dir) in Python console).
For example you have an object person, that has several attributes: name, gender, etc.
You access these attributes (be it methods or data objects) usually writing: person.name, person.gender, person.the_method(), etc.
But what if you don't know the attribute's name at the time you write the program? For example you have attribute's name stored in a variable called attr_name.
if
attr_name = 'gender'
then, instead of writing
gender = person.gender
you can write
gender = getattr(person, attr_name)
Some practice:
Python 3.4.0 (default, Apr 11 2014, 13:05:11)
>>> class Person():
... name = 'Victor'
... def say(self, what):
... print(self.name, what)
...
>>> getattr(Person, 'name')
'Victor'
>>> attr_name = 'name'
>>> person = Person()
>>> getattr(person, attr_name)
'Victor'
>>> getattr(person, 'say')('Hello')
Victor Hello
getattr will raise AttributeError if attribute with the given name does not exist in the object:
>>> getattr(person, 'age')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Person' object has no attribute 'age'
But you can pass a default value as the third argument, which will be returned if such attribute does not exist:
>>> getattr(person, 'age', 0)
0
You can use getattr along with dir to iterate over all attribute names and get their values:
>>> dir(1000)
['__abs__', '__add__', ..., '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']
>>> obj = 1000
>>> for attr_name in dir(obj):
... attr_value = getattr(obj, attr_name)
... print(attr_name, attr_value, callable(attr_value))
...
__abs__ <method-wrapper '__abs__' of int object at 0x7f4e927c2f90> True
...
bit_length <built-in method bit_length of int object at 0x7f4e927c2f90> True
...
>>> getattr(1000, 'bit_length')()
10
A practical use for this would be to find all methods whose names start with test and call them.
Similar to getattr there is setattr which allows you to set an attribute of an object having its name:
>>> setattr(person, 'name', 'Andrew')
>>> person.name # accessing instance attribute
'Andrew'
>>> Person.name # accessing class attribute
'Victor'
>>>
getattr(object, 'x') is completely equivalent to object.x.
There are only two cases where getattr can be useful.
you can't write object.x, because you don't know in advance which attribute you want (it comes from a string). Very useful for meta-programming.
you want to provide a default value. object.y will raise an AttributeError if there's no y. But getattr(object, 'y', 5) will return 5.
For me, getattr is easiest to explain this way:
It allows you to call methods based on the contents of a string instead of typing the method name.
For example, you cannot do this:
obj = MyObject()
for x in ['foo', 'bar']:
obj.x()
because x is not of the type builtin, but str. However, you CAN do this:
obj = MyObject()
for x in ['foo', 'bar']:
getattr(obj, x)()
It allows you to dynamically connect with objects based on your input. I've found it useful when dealing with custom objects and modules.
A pretty common use case for getattr is mapping data to functions.
For instance, in a web framework like Django or Pylons, getattr makes it straightforward to map a web request's URL to the function that's going to handle it. If you look under the hood of Pylons's routing, for instance, you'll see that (by default, at least) it chops up a request's URL, like:
http://www.example.com/customers/list
into "customers" and "list". Then it searches for a controller class named CustomerController. Assuming it finds the class, it creates an instance of the class and then uses getattr to get its list method. It then calls that method, passing it the request as an argument.
Once you grasp this idea, it becomes really easy to extend the functionality of a web application: just add new methods to the controller classes, and then create links in your pages that use the appropriate URLs for those methods. All of this is made possible by getattr.
Here's a quick and dirty example of how a class could fire different versions of a save method depending on which operating system it's being executed on using getattr().
import os
class Log(object):
def __init__(self):
self.os = os.name
def __getattr__(self, name):
""" look for a 'save' attribute, or just
return whatever attribute was specified """
if name == 'save':
try:
# try to dynamically return a save
# method appropriate for the user's system
return getattr(self, self.os)
except:
# bail and try to return
# a default save method
return getattr(self, '_save')
else:
return getattr(self, name)
# each of these methods could have save logic specific to
# the system on which the script is executed
def posix(self): print 'saving on a posix machine'
def nt(self): print 'saving on an nt machine'
def os2(self): print 'saving on an os2 machine'
def ce(self): print 'saving on a ce machine'
def java(self): print 'saving on a java machine'
def riscos(self): print 'saving on a riscos machine'
def _save(self): print 'saving on an unknown operating system'
def which_os(self): print os.name
Now let's use this class in an example:
logger = Log()
# Now you can do one of two things:
save_func = logger.save
# and execute it, or pass it along
# somewhere else as 1st class:
save_func()
# or you can just call it directly:
logger.save()
# other attributes will hit the else
# statement and still work as expected
logger.which_os()
Other than all the amazing answers here, there is a way to use getattr to save copious lines of code and keeping it snug. This thought came following the dreadful representation of code that sometimes might be a necessity.
Scenario
Suppose your directory structure is as follows:
- superheroes.py
- properties.py
And, you have functions for getting information about Thor, Iron Man, Doctor Strange in superheroes.py. You very smartly write down the properties of all of them in properties.py in a compact dict and then access them.
properties.py
thor = {
'about': 'Asgardian god of thunder',
'weapon': 'Mjolnir',
'powers': ['invulnerability', 'keen senses', 'vortex breath'], # and many more
}
iron_man = {
'about': 'A wealthy American business magnate, playboy, and ingenious scientist',
'weapon': 'Armor',
'powers': ['intellect', 'armor suit', 'interface with wireless connections', 'money'],
}
doctor_strange = {
'about': ' primary protector of Earth against magical and mystical threats',
'weapon': 'Magic',
'powers': ['magic', 'intellect', 'martial arts'],
}
Now, let's say you want to return capabilities of each of them on demand in superheroes.py. So, there are functions like
from .properties import thor, iron_man, doctor_strange
def get_thor_weapon():
return thor['weapon']
def get_iron_man_bio():
return iron_man['about']
def get_thor_powers():
return thor['powers']
...and more functions returning different values based on the keys and superhero.
With the help of getattr, you could do something like:
from . import properties
def get_superhero_weapon(hero):
superhero = getattr(properties, hero)
return superhero['weapon']
def get_superhero_powers(hero):
superhero = getattr(properties, hero)
return superhero['powers']
You considerably reduced the number of lines of code, functions and repetition!
Oh and of course, if you have bad names like properties_of_thor for variables , they can be made and accessed by simply doing
def get_superhero_weapon(hero):
superhero = 'properties_of_{}'.format(hero)
all_properties = getattr(properties, superhero)
return all_properties['weapon']
NOTE: For this particular problem, there can be smarter ways to deal with the situation, but the idea is to give an insight about using getattr in right places to write cleaner code.
# getattr
class hithere():
def french(self):
print 'bonjour'
def english(self):
print 'hello'
def german(self):
print 'hallo'
def czech(self):
print 'ahoj'
def noidea(self):
print 'unknown language'
def dispatch(language):
try:
getattr(hithere(),language)()
except:
getattr(hithere(),'noidea')()
# note, do better error handling than this
dispatch('french')
dispatch('english')
dispatch('german')
dispatch('czech')
dispatch('spanish')
I sometimes use getattr(..) to lazily initialise attributes of secondary importance just before they are used in the code.
Compare the following:
class Graph(object):
def __init__(self):
self.n_calls_to_plot = 0
#...
#A lot of code here
#...
def plot(self):
self.n_calls_to_plot += 1
To this:
class Graph(object):
def plot(self):
self.n_calls_to_plot = 1 + getattr(self, "n_calls_to_plot", 0)
The advantage of the second way is that n_calls_to_plot only appears around the place in the code where it is used. This is good for readability, because (1) you can immediately see what value it starts with when reading how it's used, (2) it doesn't introduce a distraction into the __init__(..) method, which ideally should be about the conceptual state of the class, rather than some utility counter that is only used by one of the function's methods for technical reasons, such as optimisation, and has nothing to do with the meaning of the object.
Quite frequently when I am creating an XML file from data stored in a class I would frequently receive errors if the attribute didn't exist or was of type None. In this case, my issue wasn't not knowing what the attribute name was, as stated in your question, but rather was data ever stored in that attribute.
class Pet:
def __init__(self):
self.hair = None
self.color = None
If I used hasattr to do this, it would return True even if the attribute value was of type None and this would cause my ElementTree set command to fail.
hasattr(temp, 'hair')
>>True
If the attribute value was of type None, getattr would also return it which would cause my ElementTree set command to fail.
c = getattr(temp, 'hair')
type(c)
>> NoneType
I use the following method to take care of these cases now:
def getRealAttr(class_obj, class_attr, default = ''):
temp = getattr(class_obj, class_attr, default)
if temp is None:
temp = default
elif type(temp) != str:
temp = str(temp)
return temp
This is when and how I use getattr.
Another use of getattr() in implementing a switch statement in Python. It uses both reflection to get the case type.
import sys
class SwitchStatement(object):
""" a class to implement switch statement and a way to show how to use gettattr in Pythion"""
def case_1(self):
return "value for case_1"
def case_2(self):
return "value for case_2"
def case_3(self):
return "value for case_3"
def case_4(self):
return "value for case_4"
def case_value(self, case_type=1):
"""This is the main dispatchmethod, that uses gettattr"""
case_method = 'case_' + str(case_type)
# fetch the relevant method name
# Get the method from 'self'. Default to a lambda.
method = getattr(self, case_method, lambda: "Invalid case type")
# Call the method as we return it
return method()
def main(_):
switch = SwitchStatement()
print swtich.case_value(_)
if __name__ == '__main__':
main(int(sys.argv[1]))
setattr()
We use setattr to add an attribute to our class instance. We pass the class instance, the attribute name, and the value.
getattr()
With getattr we retrive these values
For example
Employee = type("Employee", (object,), dict())
employee = Employee()
# Set salary to 1000
setattr(employee,"salary", 1000 )
# Get the Salary
value = getattr(employee, "salary")
print(value)
I think this example is self explanatory. It runs the method of first parameter, whose name is given in the second parameter.
class MyClass:
def __init__(self):
pass
def MyMethod(self):
print("Method ran")
# Create an object
object = MyClass()
# Get all the methods of a class
method_list = [func for func in dir(MyClass) if callable(getattr(MyClass, func))]
# You can use any of the methods in method_list
# "MyMethod" is the one we want to use right now
# This is the same as running "object.MyMethod()"
getattr(object,'MyMethod')()
It is also clarifying from https://www.programiz.com/python-programming/methods/built-in/getattr
class Person:
age = 23
name = "Adam"
person = Person()
print('The age is:', getattr(person, "age"))
print('The age is:', person.age)
The age is: 23
The age is: 23
class Person:
age = 23
name = "Adam"
person = Person()
# when default value is provided
print('The sex is:', getattr(person, 'sex', 'Male'))
# when no default value is provided
print('The sex is:', getattr(person, 'sex'))
The sex is: Male
AttributeError: 'Person' object has no attribute 'sex'
I have tried in Python2.7.17
Some of the fellow folks already answered. However I have tried to call
getattr(obj, 'set_value') and this didn't execute the set_value method, So i changed to getattr(obj, 'set_value')() --> This helps to invoke the same.
Example Code:
Example 1:
class GETATT_VERIFY():
name = "siva"
def __init__(self):
print "Ok"
def set_value(self):
self.value = "myself"
print "oooh"
obj = GETATT_VERIFY()
print getattr(GETATT_VERIFY, 'name')
getattr(obj, 'set_value')()
print obj.value
I am trying to automatically create some SQL tables from the definition of some Python classes, I tried using dir() but since it returns a Python Dictionary, it's not ordered so the definition order of the class members is lost.
Reading on the internet I found the following here
class OrderedClass(type):
#classmethod
def __prepare__(metacls, name, bases, **kwds):
return collections.OrderedDict()
def __new__(cls, name, bases, namespace, **kwds):
result = type.__new__(cls, name, bases, dict(namespace))
result.members = tuple(namespace)
return result
class A(metaclass=OrderedClass):
def one(self): pass
def two(self): pass
def three(self): pass
def four(self): pass
>>> A.members
('__module__', 'one', 'two', 'three', 'four')
I successfuly implemented a copy of it, and it appears to be doing what it should except that it's only saving the methods in the members variable, and I need to have also the class member variables.
Question:
How could I get a list of the member variables preserving their definition order?, I don't care about class methods, and I am actually ignoring them.
Note: The reason why the order is important is because the tables will have constraints that reference some of the table columns, and they must go after defining the column, but they are appearing before.
Edit: This is a sample class in my real program
class SQLTable(type):
#classmethod
def __prepare__(metacls, name, bases, **kwds):
return OrderedDict()
def __new__(cls, name, bases, namespace, **kwds):
result = type.__new__(cls, name, bases, dict(namespace))
result.members = tuple(namespace)
return result
class AreaFisicoAmbiental(metaclass = SQLTable):
def __init__(self, persona, datos):
# edificacion
self.persona = persona
self.tipoEdificacion = datos[0]
self.tipoDeParedes = datos[1]
self.detallesTipoDeParedes = datos[2]
self.tipoDeTecho = datos[3]
self.detallesTipoDeTecho = datos[4]
self.tipoDePiso = datos[5]
self.detallesTipoDePiso = datos[6]
# ambientes
self.problemaDeInfraestructura = datos[7]
self.detallesProblemaDeInfraestructura = datos[9]
self.condicionDeTenencia = datos[10]
self.detallesCondicionDeTenencia = datos[11]
self.sala = toBool(datos[12])
self.comedor = toBool(datos[13])
self.baño = toBool(datos[14])
self.porche = toBool(datos[15])
self.patio = toBool(datos[16])
self.lavandero = toBool(datos[17])
self.habitaciones = toInt(datos[19])
# servicios básicos
self.aguasServidas = toBool(datos[21])
self.aguaPotable = toBool(datos[22])
self.luz = toBool(datos[23])
self.gas = datos[24]
self.internet = toBool(datos[25])
Doing
print(AreaFisicoAmbiental.members)
Outputs:
('__module__', '__qualname__', '__init__')
Variable names are in spanish because their names will be used as the table column names, and also as the labels for a web application that will be generated from the database structure.
I know that Django does something like this, but I already have my database inspector which does the opposite thing, so know I need a Django like functionality to use my generator.
Updated
As I commented, I think you're probably confusing instance attributes with class attributes and really want to keep track of the latter. Instance attributes are dynamic and can be added, changed, or removed at any time, so trying to do this with a metaclass like shown in your question won't work (and different instances may have a different group of them defined).
You may be able to keep track of their creation and deletion by overloading a couple of the class's special methods, namely __setattr__() and __delattr__() and storing their effects in a private data member which is an OrderedSet. Do so will keep track of what they are and preserve the order in which they were created.
Both of these methods will need to be careful not to operate upon the private data member itself.
That said, here's something illustrating such an implementation:
# -*- coding: iso-8859-1 -*-
# from http://code.activestate.com/recipes/576694
from orderedset import OrderedSet
class AreaFisicoAmbiental(object):
def __init__(self, persona, datos):
self._members = OrderedSet()
self.persona = persona
self.tipoEdificacion = datos[0]
self.tipoDeParedes = datos[1]
def __setattr__(self, name, value):
object.__setattr__(self, name, value)
if name != '_members':
self._members.add(name)
def __delattr__(self, name):
if name != '_members':
object.__delattr__(self, name)
self._members.discard(name)
def methodA(self, value1, value2): # add some members
self.attribute1 = value1
self.attribute2 = value2
def methodB(self):
del self.attribute1 # remove a member
if __name__ == '__main__':
a = AreaFisicoAmbiental('Martineau', ['de albañilería', 'vinilo'])
a.methodA('attribute1 will be deleted', 'but this one will be retained')
a.methodB() # deletes a.attribute1
a.attribute3 = 42 # add an attribute outside the class
print('current members of "a":')
for name in a._members:
print(' {}'.format(name))
Output:
current members of "a":
persona
tipoEdificacion
tipoDeParedes
attribute2
attribute3
A final note: It would be possible to create a metaclass that added these two methods automatically to client classes, which would make it easier to modify existing classes.
Maybe, python enum would be enough for the task. Indeed it supports stable order.
The basic implementation of DDL would look like this:
from enum import Enum
class Table1(Enum):
nombre = ''
edad = 0
sexo = True
...
then later you could do:
for prop in Table1:
print(prop)
this gives you
Table1.nombre
Table1.edad
Table1.sexo
if you need to construct a proper table definition you could use Table1.<field>.value:
>>> print(type(Table1.nombre.value))
<class 'str'>
>>> print(type(Table1.edad.value))
<class 'int'>
and so on. Using this technique you could even link some tables to others thus constructing a complete definition of a whole set of tables and their relationships.
As for data objects (e.g. a row in a table, or a row of a query results), here I think you don't any own ordering, you just need to maintain a link to a corresponding table class (from which the order can be restored, however I don't think it's such a requested option). So these classes could look like this:
class Table1Row(object):
_table = Table1
__slots__ = tuple(k.name for k Table1)
...
or simply
class ASpecificQueryResults(object):
__slots__ = (Table1.nombre.name, Table2.empresa.name,...)
probably you need a factory which would build row classes based on the query results and/or table definitions.
Edit probably the idea with __slots__ in *Row classes requires some more polish but this heavily depends on your actual needs.
P.S. Perhaps 'Table1.sexo' also should be an enum in our complicated times ;)
This question already has answers here:
Getting the name of a variable as a string
(32 answers)
Closed 3 years ago.
While building a new class object in python, I want to be able to create a default value based on the instance name of the class without passing in an extra argument. How can I accomplish this? Here's the basic pseudo-code I'm trying for:
class SomeObject():
defined_name = u""
def __init__(self, def_name=None):
if def_name == None:
def_name = u"%s" % (<INSTANCE NAME>)
self.defined_name = def_name
ThisObject = SomeObject()
print ThisObject.defined_name # Should print "ThisObject"
Well, there is almost a way to do it:
#!/usr/bin/env python
import traceback
class SomeObject():
def __init__(self, def_name=None):
if def_name == None:
(filename,line_number,function_name,text)=traceback.extract_stack()[-2]
def_name = text[:text.find('=')].strip()
self.defined_name = def_name
ThisObject = SomeObject()
print ThisObject.defined_name
# ThisObject
The traceback module allows you to peek at the code used to call SomeObject().
With a little string wrangling, text[:text.find('=')].strip() you can
guess what the def_name should be.
However, this hack is brittle. For example, this doesn't work so well:
ThisObject,ThatObject = SomeObject(),SomeObject()
print ThisObject.defined_name
# ThisObject,ThatObject
print ThatObject.defined_name
# ThisObject,ThatObject
So if you were to use this hack, you have to bear in mind that you must call SomeObject()
using simple python statement:
ThisObject = SomeObject()
By the way, as a further example of using traceback, if you define
def pv(var):
# stack is a list of 4-tuples: (filename, line number, function name, text)
# see http://docs.python.org/library/traceback.html#module-traceback
#
(filename,line_number,function_name,text)=traceback.extract_stack()[-2]
# ('x_traceback.py', 18, 'f', 'print_var(y)')
print('%s: %s'%(text[text.find('(')+1:-1],var))
then you can call
x=3.14
pv(x)
# x: 3.14
to print both the variable name and its value.
Instances don't have names. By the time the global name ThisObject gets bound to the instance created by evaluating the SomeObject constructor, the constructor has finished running.
If you want an object to have a name, just pass the name along in the constructor.
def __init__(self, name):
self.name = name
You can create a method inside your class that check all variables in the current frame and use hash() to look for the self variable.
The solution proposed here will return all the variables pointing to the instance object.
In the class below, isinstance() is used to avoid problems when applying hash(), since some objects like a numpy.array or a list, for example, are unhashable.
import inspect
class A(object):
def get_my_name(self):
ans = []
frame = inspect.currentframe().f_back
tmp = dict(frame.f_globals.items() + frame.f_locals.items())
for k, var in tmp.items():
if isinstance(var, self.__class__):
if hash(self) == hash(var):
ans.append(k)
return ans
The following test has been done:
def test():
a = A()
b = a
c = b
print c.get_my_name()
The result is:
test()
#['a', 'c', 'b']
This cannot work, just imagine this: a = b = TheMagicObjet(). Names have no effect on Values, they just point to them.
One horrible, horrible way to accomplish this is to reverse the responsibilities:
class SomeObject():
def __init__(self, def_name):
self.defined_name = def_name
globals()[def_name] = self
SomeObject("ThisObject")
print ThisObject.defined_name
If you wanted to support something other than global scope, you'd have to do something even more awful.
In Python, all data is stored in objects. Additionally, a name can be bound with an object, after which that name can be used to look up that object.
It makes no difference to the object what names, if any, it might be bound to. It might be bound to dozens of different names, or none. Also, Python does not have any "back links" that point from an object to a name.
Consider this example:
foo = 1
bar = foo
baz = foo
Now, suppose you have the integer object with value 1, and you want to work backwards and find its name. What would you print? Three different names have that object bound to them, and all are equally valid.
print(bar is foo) # prints True
print(baz is foo) # prints True
In Python, a name is a way to access an object, so there is no way to work with names directly. You could search through various name spaces until you find a name that is bound with the object of interest, but I don't recommend this.
How do I get the string representation of a variable in python?
There is a famous presentation called "Code Like a Pythonista" that summarizes this situation as "Other languages have 'variables'" and "Python has 'names'"
http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables
If you want an unique instance name for a class, try __repr__() or id(self)
class Some:
def __init__(self):
print(self.__repr__()) # = hex(id(self))
print(id(self))
It will print the memory address of the instance, which is unique.
Inspired by the answers of unutbu and Saullo Castro, I have created a more sophisticated class that can even be subclassed. It solves what was asked for in the question.
"create a default value based on the instance name of the class
without passing in an extra argument."
Here's what it does, when an instance of this class or a subclass is created:
Go up in the frame stack until the first frame which does not belong to a method of the current instance.
Inspect this frame to get the attributes self.creation_(name/file/module/function/line/text).
Perform an an additional check whether an object with name self.creation_name was actually defined in the frame's locals() namespace to make 100% sure the found creation_name is correct or raise an error otherwise.
The Code:
import traceback, threading, time
class InstanceCreationError(Exception):
pass
class RememberInstanceCreationInfo:
def __init__(self):
for frame, line in traceback.walk_stack(None):
varnames = frame.f_code.co_varnames
if varnames is ():
break
if frame.f_locals[varnames[0]] not in (self, self.__class__):
break
# if the frame is inside a method of this instance,
# the first argument usually contains either the instance or
# its class
# we want to find the first frame, where this is not the case
else:
raise InstanceCreationError("No suitable outer frame found.")
self._outer_frame = frame
self.creation_module = frame.f_globals["__name__"]
self.creation_file, self.creation_line, self.creation_function, \
self.creation_text = \
traceback.extract_stack(frame, 1)[0]
self.creation_name = self.creation_text.split("=")[0].strip()
super().__init__()
threading.Thread(target=self._check_existence_after_creation).start()
def _check_existence_after_creation(self):
while self._outer_frame.f_lineno == self.creation_line:
time.sleep(0.01)
# this is executed as soon as the line number changes
# now we can be sure the instance was actually created
error = InstanceCreationError(
"\nCreation name not found in creation frame.\ncreation_file: "
"%s \ncreation_line: %s \ncreation_text: %s\ncreation_name ("
"might be wrong): %s" % (
self.creation_file, self.creation_line, self.creation_text,
self.creation_name))
nameparts = self.creation_name.split(".")
try:
var = self._outer_frame.f_locals[nameparts[0]]
except KeyError:
raise error
finally:
del self._outer_frame
# make sure we have no permament inter frame reference
# which could hinder garbage collection
try:
for name in nameparts[1:]: var = getattr(var, name)
except AttributeError:
raise error
if var is not self: raise error
def __repr__(self):
return super().__repr__()[
:-1] + " with creation_name '%s'>" % self.creation_name
A simple example:
class MySubclass(RememberInstanceCreationInfo):
def __init__(self):
super().__init__()
def print_creation_info(self):
print(self.creation_name, self.creation_module, self.creation_function,
self.creation_line, self.creation_text, sep=", ")
instance = MySubclass()
instance.print_creation_info()
#out: instance, __main__, <module>, 68, instance = MySubclass()
If the creation name cannot be determined properly an error is raised:
variable, another_instance = 2, MySubclass()
# InstanceCreationError:
# Creation name not found in creation frame.
# creation_file: /.../myfile.py
# creation_line: 71
# creation_text: variable, another_instance = 2, MySubclass()
# creation_name (might be wrong): variable, another_instance
I think that names matters if they are the pointers to any object..
no matters if:
foo = 1
bar = foo
I know that foo points to 1 and bar points to the same value 1 into the same memory space.
but supose that I want to create a class with a function that adds a object to it.
Class Bag(object):
def __init__(self):
some code here...
def addItem(self,item):
self.__dict__[somewaytogetItemName] = item
So, when I instantiate the class bag like below:
newObj1 = Bag()
newObj2 = Bag()
newObj1.addItem(newObj2)I can do this to get an attribute of newObj1:
newObj1.newObj2
The best way is really to pass the name to the constructor as in the chosen answer. However, if you REALLY want to avoid asking the user to pass the name to the constructor, you can do the following hack:
If you are creating the instance with 'ThisObject = SomeObject()' from the command line, you can get the object name from the command string in command history:
import readline
import re
class SomeObject():
def __init__(self):
cmd = readline.get_history_item(readline.get_current_history_length())
self.name = re.split('=| ',cmd)[0]
If you are creating the instance using 'exec' command, you can handle this with:
if cmd[0:4] == 'exec': self.name = re.split('\'|=| ',cmd)[1] # if command performed using 'exec'
else: self.name = re.split('=| ',cmd)[0]
I am programming a simulations for single neurons. Therefore I have to handle a lot of Parameters. Now the Idea is that I have two classes, one for a SingleParameter and a Collection of parameters. I use property() to access the parameter value easy and to make the code more readable. This works perfect for a sinlge parameter but I don't know how to implement it for the collection as I want to name the property in Collection after the SingleParameter. Here an example:
class SingleParameter(object):
def __init__(self, name, default_value=0, unit='not specified'):
self.name = name
self.default_value = default_value
self.unit = unit
self.set(default_value)
def get(self):
return self._v
def set(self, value):
self._v = value
v = property(fget=get, fset=set, doc='value of parameter')
par1 = SingleParameter(name='par1', default_value=10, unit='mV')
par2 = SingleParameter(name='par2', default_value=20, unit='mA')
# par1 and par2 I can access perfectly via 'p1.v = ...'
# or get its value with 'p1.v'
class Collection(object):
def __init__(self):
self.dict = {}
def __getitem__(self, name):
return self.dict[name] # get the whole object
# to get the value instead:
# return self.dict[name].v
def add(self, parameter):
self.dict[parameter.name] = parameter
# now comes the part that I don't know how to implement with property():
# It shoule be something like
# self.__dict__[parameter.name] = property(...) ?
col = Collection()
col.add(par1)
col.add(par2)
col['par1'] # gives the whole object
# Now here is what I would like to get:
# col.par1 -> should result like col['par1'].v
# col.par1 = 5 -> should result like col['par1'].v = 5
Other questions that I put to understand property():
Why do managed attributes just work for class attributes and not for instance attributes in python?
How can I assign a new class attribute via __dict__ in python?
Look at built-in functions getattr and setattr. You'll probably be a lot happier.
Using the same get/set functions for both classes forces you into an ugly hack with the argument list. Very sketchy, this is how I would do it:
In class SingleParameter, define get and set as usual:
def get(self):
return self._s
def set(self, value):
self._s = value
In class Collection, you cannot know the information until you create the property, so you define the metaset/metaget function and particularize them only later with a lambda function:
def metaget(self, par):
return par.s
def metaset(self, value, par):
par.s = value
def add(self, par):
self[par.name] = par
setattr(Collection, par.name,
property(
fget=lambda x : Collection.metaget(x, par),
fset=lambda x, y : Collection.metaset(x,y, par))
Properties are meant to dynamically evaluate attributes or to make them read-only. What you need is customizing attribute access. __getattr__ and __setattr__ do that really fine, and there's also __getattribute__ if __getattr__ is not enough.
See Python docs on customizing attribute access for details.
Have you looked at the traits package? It seems that you are reinventing the wheel here with your parameter classes. Traits also have additional features that might be useful for your type of application (incidently I know a person that happily uses traits in neural simulations).
Now I implemented a solution with set-/getattr:
class Collection(object):
...
def __setattr__(self, name, value):
if 'dict' in self.__dict__:
if name in self.dict:
self[name].v = value
else:
self.__dict__[name] = value
def __getattr__(self, name):
return self[name].v
There is one thing I quite don't like that much: The attributes are not in the __dict__. And if I have them there as well I would have a copy of the value - which can be dangerous...
Finally I succeded to implement the classes with property(). Thanks a lot for the advice. It took me quite a bit to work it out - but I can promise you that this exercise helps you to understand better pythons OOP.
I implemented it also with __getattr__ and __setattr__ but still don't know the advantages and disadvantages to the property-solution. But this seems to be worth another question. The property-solutions seems to be quit clean.
So here is the code:
class SingleParameter(object):
def __init__(self, name, default_value=0, unit='not specified'):
self.name = name
self.default_value = default_value
self.unit = unit
self.set(default_value)
def get(*args):
self = args[0]
print "get(): "
print args
return self._v
def set(*args):
print "set(): "
print args
self = args[0]
value = args[-1]
self._v = value
v = property(fget=get, fset=set, doc='value of parameter')
class Collection(dict):
# inheriting from dict saves the methods: __getitem__ and __init__
def add(self, par):
self[par.name] = par
# Now here comes the tricky part.
# (Note: this property call the get() and set() methods with one
# more argument than the property of SingleParameter)
setattr(Collection, par.name,
property(fget=par.get, fset=par.set))
# Applying the classes:
par1 = SingleParameter(name='par1', default_value=10, unit='mV')
par2 = SingleParameter(name='par2', default_value=20, unit='mA')
col = Collection()
col.add(par1)
col.add(par2)
# Setting parameter values:
par1.v = 13
col.par1 = 14
# Getting parameter values:
par1.v
col.par1
# checking identity:
par1.v is col.par1
# to access the whole object:
col['par1']
As I am new I am not sure how to move on:
how to treat follow up questions (like this itself):
get() is seems to be called twice - why?
oop-design: property vs. "__getattr__ & __setattr__" - when should I use what?
is it rude to check the own answer to the own question as accepted?
is it recommended to rename the title in order to put correlated questions or questions elaborated with the same example into the same context?
Other questions that I put to understand property():
Why do managed attributes just work for class attributes and not for instance attributes in python?
How can I assign a new class attribute via __dict__ in python?
I have a class that does something similar, but I did the following in the collection object:
setattr(self, par.name, par.v)