While programming with python I am often confused with the following ambiguity:
should it be: function(a) or a.function(). Although the question is too general and can someone tell me which situation happens when?
Your confusion potentially stems from how python defines instance methods...
class Person
def speak(self, message):
print message
Looking at that definition I can see how someone would think you have to pass a Person as the first argument.
but it is just python's way to make self, the current instance of the person, available to the method.
so the proper way would be
person_instance = Person()
person_instance.speak("This is a message")
When you call it like a.function() you are calling a method named "function" from the object named "a".
If you don't know the difference between a method and a function/procedure, you need to study Object Oriented Programming concepts.
Related
I want to use a method from another class.
Neither inheritance nor delegation is a good choice (to my understanding) because the existing class is too complicated to override and too expensive to instanciate.
Note that modifying the existing class is not allowed (legacy project, you know).
I came up with a way:
class Old:
def a(self):
print('Old.a')
class Mine:
b = Old.a
and it shows
>>> Mine().b()
Old.a
>>> Mine().b
<bound method Old.a of <__main__.Mine object at 0x...>>
It seems fine.
And I tried with some more complicated cases including property modification (like self.foo = 'bar'), everything seems okay.
My question:
What is actually happening when I define methods like that?
Will that safely do the trick for my need mentioned above?
Explanation
What's happening is that you are defining a callable class property of class Mine called b. However, this works:
m = Mine()
m.b()
But this won't:
Mine.b()
Why doesn't the second way work?
When you call a function of a class, python expects the first argument to be the actual object upon which the function was called. When you do this, the self argument is automatically passed into the function behind the scenes. Since we called Mine.b() without an instantiated instance of any object, no self was passed into b().
Will this "do the trick"?
As for whether this will do the trick, that depends.
As long as Mine can behave the same way as Old, python won't complain. This is because the python interpreter does not care about the "type" of self. As long as it walks like a duck and quacks like a duck, it's a duck (see duck typing). However, can you guarantee this? What if someone goes and changes the implementation of Old.a. Most of the time, as a client of another system we have no say when the private implementation of functions change.
A simpler solution might be to pull out the functionality you are missing into a separate module. Yes, there is some code duplication but at least you can be confident the code won't change from under you.
Ultimately, if you can guarantee the behavior of Old and Mine will be similar enough for the purposes of Old.a, python really shouldn't care.
I am assuming this question has been asked a million times already but I can't seem to make sense of a few things so please bear with me here. I am trying to make do generic inheritance in Python. This is what I want to accomplish: I have a method that takes in a generic type and it returns a class that has been inherited from that parent class
this is what the code looks like
def make_foo(parent):
class Relationship(parent):
# def __init__(self):
#staticmethod
def should_see_this_method(self):
print("Hello here")
return True
return Relationship
Now this is the piece of code I am have
NewType = make_relationship(str)
another_test: NewType = "Hello"
another_test.should_see_this_method()
another_test.capitalize()
Now I am getting AttributeError: 'str' object has no attribute 'should_see_this_method'
I am not sure if this is anti pattern or not but I am just curios to know how I can do this.
thanks
This line:
another_test: NewType = "Hello"
doesn't do what you think it does.
This is a type hint. Hints are used by static type checkers, linters, and the like to check if your code has obvious bugs or is being used incorrectly. It helps you at "compile time" to catch things that are possible sources of errors, but it has no impact on the runtime behavior of the code.
Importantly, it does not construct an object of type NewType. It constructs a str. You can see this easily by calling type(another_test), which indicates this is a str. (It's also in the message of the AttributeError in your question.)
To actually construct that object, you have to do the usual thing:
>>> another_test = NewType("Hello")
>>> isinstance(another_test, NewType)
True
An unrelated problem in your code: staticmethods should not take self as the first argument. They are not bound to any instance. You'll see an error once you actually get to the line which calls the method.
I am new to programming so when I wanted a command to turn a hard-coded function's name into a string, I looked it up and started using the built-in __name__ function. The problem is that I don't think I understand how __name__ retrieves the wanted name. I know it has something to do with what is currently visible with local() or dir() but that's about it... (my own research on this topic has been kinda of hard to understand for me) As a result, I stumbled across an error that I don't know how to solve.
Here is some code that recreates my error:
class Abc:
#staticmethod
def my_static_func():
return
def my_method(self):
return
class_list = [my_static_func]
method_list = [my_method]
#These calls work
Abc.my_static_func.__name__
Abc.my_method.__name__
Abc.method_list[0].__name__
#But This call raises an AttributeError
Abc.class_list[0].__name__
I get this error message:
AttributeError: 'staticmethod' object has no attribute '__name__'
So why is that when I put my static method into a list and then try to get the function's name from the list it doesn't work? Please forgive me if this question is dumb. As you can clearly see, I don't understand the underlying principle of how __name__ works (and other stuff that I just don't know about to even give these topics names!). An answer would be nice but a reference to some docs would be also welcome.
With this code
#staticmethod
def my_static_method(...):
....
The function my_static_method is wrapped in a staticmethod. When you access a staticmethod from a class some magic happens to the staticmethod and you actually get a proper function back. Thats why you can access its __name__
Putting the staticmethod in a list and accessing it from that list prevents the magic from happening and you get the staticmethod object back.
Abc.my_static_func # this is a function
Abc.class_list[0] # this is a staticmethod object
As a staticmethod does not have a __name__ you get an AttibuteError when accessing its __name__.
To solve your problem you could get the underlying function from the staticmethod
Abc.class_list[0].__func__ # the function
Abc.class_list[0].__func__.__name__ # its name
To find out more about the "magic" that happens when you access attributes/methods from a class/object look up descriptors
I was sure that there'd be an answer to this question somewhere on stack overflow, but I haven't been able to find one; most of them are in regards to passing functions, and not methods, as arguments to functions.
I'm currently working with Python 2.7.5 and I'm trying to define a function like this:
def func(object, method):
object.method()
that when called like so:
some_object_instance = SomeObject()
func(some_object_instance, SomeObject.some_object_method)
using a class defined like this:
class SomeObject:
def some_object_method(self):
# do something
is basically equivalent to doing this:
some_object_instance.some_object_method()
I, however, keep getting an attribute error--something along the lines of
'SomeObject' has no attribute 'method'
I was under the impression that I could legally pass methods as arguments and have them evaluate correctly when used in the aforementioned manner. What am I missing?
That's not the way method calling works. The foo.bar syntax looks for a method named bar on the foo object. If you already have the method, just call it:
def func(object, method):
method(object)
func(some_object_instance, SomeObject.some_object_method)
SomeObject.some_object_method is what's called an "unbound method": it's a method object without a self bound into it, so you have to explicitly pass the self to it.
This might make more sense with a concrete example:
>>> s = 'ABC'
>>> s_lower = s.lower # bound method
>>> s_lower()
'abc'
>>> str_lower = str.lower # unbound method
>>> str_lower(s)
'abc'
By comparison, some_object_instance.some_object_method is a "bound method", you can just call it as-is, and some_object_instance is already "bound in" as the self argument:
def func2(method):
method()
func2(some_object_instance.some_object_method)
Unbound methods aren't explained in detail the tutorial; they're covered in the section on bound methods. So you have to go to the reference manual for documentation (in [standard type hierarchy] (https://docs.python.org/2/reference/datamodel.html#the-standard-type-hierarchy), way down in the subsection "User-defined methods"), which can be a little bit daunting for novices.
Personally, I didn't really get this until I learned how it worked under the covers. About a year ago, I wrote a blog post How methods work to try to explain it to someone else (but in Python 3.x terms, which is slightly different); it may help. To really get it, you have to get through the Descriptor HOWTO, which may take a few read-throughs and a lot of playing around in the interactive interpreter before it really clicks, but hopefully you can understand the basic concepts behind methods before getting to that point.
Since you are passing an unbound method to the function, you need to call it as:
method(object)
Or better pass the name of the method as string and then use getattr:
getattr(object, method)()
I'm new in programming so please don't kill me for asking stupid questions.
I've been trying to understand all that class business in Python and I got to the point where could not find answer for my question just by google it.
In my program I need to call a class from within other class based on string returned by function. I found two solutions: one by using getattr() and second one by using globals() / locals().
Decided to go for second solution and got it working but I'm really don't understand how it's working.
So there is the code example:
class Test(object):
def __init__(self):
print "WORKS!"
room = globals()['Test']
room()
type(room()) gives:
<class '__main__.Test'>
type(room) gives:
<type 'type'> # What????
It looks like room() is a class object, but shouldn't that be room instead of room()?
Please help me because it is a little bit silly if I write a code which I don't understand myself.
What happens here is the following:
class Test(object):
def __init__(self):
print "WORKS!"
room = globals()['Test']
Here you got Test as room the way you wanted. Verify this:
room is Test
should give True.
type(room()) gives:
<class '__main__.Test'>
You do one step an go it backwards: room() returns the same as Test() would - an instance of that class. type() "undoes" this step resp. gets the type of the object - this is, of course, Test.
type(room) gives:
<type 'type'> # What????
Of course - it is the type of a (new style) class. The same as type(Test).
Be aware, however, that for
In my program I need to call a class from within other class based on string returned by function. I found two solutions: one by using getattr() and second one by using globals() / locals().
it could be better to create an explicitly separate dict. Here you have full control over which objects/classes/... are allowed in that context and which are not.
First of all, I'd go with getattr instead.
In your example, room equals Test and is a class. Its type is type.
When you call room(), you instantiate Test, so room() evaluates to an instance of Test, whose type is Test.
Classes are objects too, in Python. All this does:
class Test(object):
def __init__(self):
print "WORKS!"
is create a class object and bind it to the name Test. Much as this:
x = []
creates a list object and binds it to the name x.
Test() isn't magic syntax for creating an instance. The Test is perfectly ordinary variable lookup, and the () is perfectly ordinary "call with empty arguments". It just so happens that calling a class will create an instance of that class.
If follows then that your problem of instantiating a class chosen based on having the name of the class as a string boils down to the much simpler problem of finding an object stored in a variable. It's exactly the same problem as getting that list bound to the name x, given the string "x". Once you've got a reference to the class in any old variable, you can simply call it to create your instance.
globals() returns a dictionary mapping the names of globals to their values. So globals()['Test'] will get you the class Test just as easily as globals()['x'] will get you the list. However it's usually not considered great style to use globals() like this; your module probably contains a large number of callables (including a bunch imported from other modules) that you don't want to be accidentally invoked if the function can be made to return their name. Given that classes are just ordinary objects, you can put them in a dictionary of your own making:
classes = {
'Test': Test,
'SomethingElse': Something,
...
}
This involves a bit more typing, but it's also easier to see what the intended usage is, and it gives you a bit more flexibility, since you can also easily pass this dictionary to other modules and have the instantiation take place elsewhere (you could do that with globals(), but then you're getting very weird).
Now, for the type(room) being type. Again, this is just a simple consequence of the fact that classes themselves are also objects. If a class is an object, then it should also be an instance of some class. What class is that? type, the "type of types". Much as any class defines the common behaviour of all its instances, the class type defines the common behaviour of all classes.
And just to make your brain hurt, type is an instance of itself (since type is also a class, and type is the class of classes). And it's a subclass of object (since all type instances are object instances, but not all object instances are type instances), and also an instance of object (since object is the root class of which everything is an instance).
You can generally ignore type as an advanced topic, however. :)