How can I pass an integer by reference in Python?
I want to modify the value of a variable that I am passing to the function. I have read that everything in Python is pass by value, but there has to be an easy trick. For example, in Java you could pass the reference types of Integer, Long, etc.
How can I pass an integer into a function by reference?
What are the best practices?
It doesn't quite work that way in Python. Python passes references to objects. Inside your function you have an object -- You're free to mutate that object (if possible). However, integers are immutable. One workaround is to pass the integer in a container which can be mutated:
def change(x):
x[0] = 3
x = [1]
change(x)
print x
This is ugly/clumsy at best, but you're not going to do any better in Python. The reason is because in Python, assignment (=) takes whatever object is the result of the right hand side and binds it to whatever is on the left hand side *(or passes it to the appropriate function).
Understanding this, we can see why there is no way to change the value of an immutable object inside a function -- you can't change any of its attributes because it's immutable, and you can't just assign the "variable" a new value because then you're actually creating a new object (which is distinct from the old one) and giving it the name that the old object had in the local namespace.
Usually the workaround is to simply return the object that you want:
def multiply_by_2(x):
return 2*x
x = 1
x = multiply_by_2(x)
*In the first example case above, 3 actually gets passed to x.__setitem__.
Most cases where you would need to pass by reference are where you need to return more than one value back to the caller. A "best practice" is to use multiple return values, which is much easier to do in Python than in languages like Java.
Here's a simple example:
def RectToPolar(x, y):
r = (x ** 2 + y ** 2) ** 0.5
theta = math.atan2(y, x)
return r, theta # return 2 things at once
r, theta = RectToPolar(3, 4) # assign 2 things at once
Not exactly passing a value directly, but using it as if it was passed.
x = 7
def my_method():
nonlocal x
x += 1
my_method()
print(x) # 8
Caveats:
nonlocal was introduced in python 3
If the enclosing scope is the global one, use global instead of nonlocal.
Maybe it's not pythonic way, but you can do this
import ctypes
def incr(a):
a += 1
x = ctypes.c_int(1) # create c-var
incr(ctypes.ctypes.byref(x)) # passing by ref
Really, the best practice is to step back and ask whether you really need to do this. Why do you want to modify the value of a variable that you're passing in to the function?
If you need to do it for a quick hack, the quickest way is to pass a list holding the integer, and stick a [0] around every use of it, as mgilson's answer demonstrates.
If you need to do it for something more significant, write a class that has an int as an attribute, so you can just set it. Of course this forces you to come up with a good name for the class, and for the attribute—if you can't think of anything, go back and read the sentence again a few times, and then use the list.
More generally, if you're trying to port some Java idiom directly to Python, you're doing it wrong. Even when there is something directly corresponding (as with static/#staticmethod), you still don't want to use it in most Python programs just because you'd use it in Java.
Maybe slightly more self-documenting than the list-of-length-1 trick is the old empty type trick:
def inc_i(v):
v.i += 1
x = type('', (), {})()
x.i = 7
inc_i(x)
print(x.i)
A numpy single-element array is mutable and yet for most purposes, it can be evaluated as if it was a numerical python variable. Therefore, it's a more convenient by-reference number container than a single-element list.
import numpy as np
def triple_var_by_ref(x):
x[0]=x[0]*3
a=np.array([2])
triple_var_by_ref(a)
print(a+1)
output:
7
The correct answer, is to use a class and put the value inside the class, this lets you pass by reference exactly as you desire.
class Thing:
def __init__(self,a):
self.a = a
def dosomething(ref)
ref.a += 1
t = Thing(3)
dosomething(t)
print("T is now",t.a)
In Python, every value is a reference (a pointer to an object), just like non-primitives in Java. Also, like Java, Python only has pass by value. So, semantically, they are pretty much the same.
Since you mention Java in your question, I would like to see how you achieve what you want in Java. If you can show it in Java, I can show you how to do it exactly equivalently in Python.
class PassByReference:
def Change(self, var):
self.a = var
print(self.a)
s=PassByReference()
s.Change(5)
class Obj:
def __init__(self,a):
self.value = a
def sum(self, a):
self.value += a
a = Obj(1)
b = a
a.sum(1)
print(a.value, b.value)// 2 2
In Python, everything is passed by value, but if you want to modify some state, you can change the value of an integer inside a list or object that's passed to a method.
integers are immutable in python and once they are created we cannot change their value by using assignment operator to a variable we are making it to point to some other address not the previous address.
In python a function can return multiple values we can make use of it:
def swap(a,b):
return b,a
a,b=22,55
a,b=swap(a,b)
print(a,b)
To change the reference a variable is pointing to we can wrap immutable data types(int, long, float, complex, str, bytes, truple, frozenset) inside of mutable data types (bytearray, list, set, dict).
#var is an instance of dictionary type
def change(var,key,new_value):
var[key]=new_value
var =dict()
var['a']=33
change(var,'a',2625)
print(var['a'])
I already read How to get a function name as a string?.
How can I do the same for a variable? As opposed to functions, Python variables do not have the __name__ attribute.
In other words, if I have a variable such as:
foo = dict()
foo['bar'] = 2
I am looking for a function/attribute, e.g. retrieve_name() in order to create a DataFrame in Pandas from this list, where the column names are given by the names of the actual dictionaries:
# List of dictionaries for my DataFrame
list_of_dicts = [n_jobs, users, queues, priorities]
columns = [retrieve_name(d) for d in list_of_dicts]
With Python 3.8 one can simply use f-string debugging feature:
>>> foo = dict()
>>> f'{foo=}'.split('=')[0]
'foo'
One drawback of this method is that in order to get 'foo' printed you have to add f'{foo=}' yourself. In other words, you already have to know the name of the variable. In other words, the above code snippet is exactly the same as just
>>> 'foo'
Even if variable values don't point back to the name, you have access to the list of every assigned variable and its value, so I'm astounded that only one person suggested looping through there to look for your var name.
Someone mentioned on that answer that you might have to walk the stack and check everyone's locals and globals to find foo, but if foo is assigned in the scope where you're calling this retrieve_name function, you can use inspect's current frame to get you all of those local variables.
My explanation might be a little bit too wordy (maybe I should've used a "foo" less words), but here's how it would look in code (Note that if there is more than one variable assigned to the same value, you will get both of those variable names):
import inspect
x, y, z = 1, 2, 3
def retrieve_name(var):
callers_local_vars = inspect.currentframe().f_back.f_locals.items()
return [var_name for var_name, var_val in callers_local_vars if var_val is var]
print(retrieve_name(y))
If you're calling this function from another function, something like:
def foo(bar):
return retrieve_name(bar)
foo(baz)
And you want the baz instead of bar, you'll just need to go back a scope further. This can be done by adding an extra .f_back in the caller_local_vars initialization.
See an example here: ideone
The only objects in Python that have canonical names are modules, functions, and classes, and of course there is no guarantee that this canonical name has any meaning in any namespace after the function or class has been defined or the module imported. These names can also be modified after the objects are created so they may not always be particularly trustworthy.
What you want to do is not possible without recursively walking the tree of named objects; a name is a one-way reference to an object. A common or garden-variety Python object contains no references to its names. Imagine if every integer, every dict, every list, every Boolean needed to maintain a list of strings that represented names that referred to it! It would be an implementation nightmare, with little benefit to the programmer.
TL;DR
Use the Wrapper helper from python-varname:
from varname.helpers import Wrapper
foo = Wrapper(dict())
# foo.name == 'foo'
# foo.value == {}
foo.value['bar'] = 2
For list comprehension part, you can do:
n_jobs = Wrapper(<original_value>)
users = Wrapper(<original_value>)
queues = Wrapper(<original_value>)
priorities = Wrapper(<original_value>)
list_of_dicts = [n_jobs, users, queues, priorities]
columns = [d.name for d in list_of_dicts]
# ['n_jobs', 'users', 'queues', 'priorities']
# REMEMBER that you have to access the <original_value> by d.value
I am the author of the python-varname package. Please let me know if you have any questions or you can submit issues on Github.
The long answer
Is it even possible?
Yes and No.
We are retrieving the variable names at runtime, so we need a function to be called to enable us to access the previous frames to retrieve the variable names. That's why we need a Wrapper there. In that function, at runtime, we are parsing the source code/AST nodes in the previous frames to get the exact variable name.
However, the source code/AST nodes in the previous frames are not always available, or they could be modified by other environments (e.g: pytest's assert statement). One simple example is that the codes run via exec(). Even though we are still able to retrieve some information from the bytecode, it needs too much effort and it is also error-prone.
How to do it?
First of all, we need to identify which frame the variable is given. It's not always simply the direct previous frame. For example, we may have another wrapper for the function:
from varname import varname
def func():
return varname()
def wrapped():
return func()
x = wrapped()
In the above example, we have to skip the frame inside wrapped to get to the right frame x = wrapped() so that we are able to locate x. The arguments frame and ignore of varname allow us to skip some of these intermediate frames. See more details in the README file and the API docs of the package.
Then we need to parse the AST node to locate where the variable is assigned value (function call) to. It's not always just a simple assignment. Sometimes there could be complex AST nodes, for example, x = [wrapped()]. We need to identify the correct assignment by traversing the AST tree.
How reliable is it?
Once we identify the assignment node, it is reliable.
varname is all depending on executing package to look for the node. The node executing detects is ensured to be the correct one (see also this).
It partially works with environments where other AST magics apply, including pytest, ipython, macropy, birdseye, reticulate with R, etc. Neither executing nor varname is 100% working with those environments.
Do we need a package to do it?
Well, yes and no, again.
If your scenario is simple, the code provided by #juan Isaza or #scohe001 probably is enough for you to work with the case where a variable is defined at the direct previous frame and the AST node is a simple assignment. You just need to go one frame back and retrieve the information there.
However, if the scenario becomes complicated, or we need to adopt different application scenarios, you probably need a package like python-varname, to handle them. These scenarios may include to:
present more friendly messages when the source code is not available or AST nodes are not accessible
skip intermediate frames (allows the function to be wrapped or called in other intermediate frames)
automatically ignores calls from built-in functions or libraries. For example: x = str(func())
retrieve multiple variable names on the left-hand side of the assignment
etc.
How about the f-string?
Like the answer provided by #Aivar Paalberg. It's definitely fast and reliable. However, it's not at runtime, meaning that you have to know it's foo before you print the name out. But with varname, you don't have to know that variable is coming:
from varname import varname
def func():
return varname()
# In external uses
x = func() # 'x'
y = func() # 'y'
Finally
python-varname is not only able to detect the variable name from an assignment, but also:
Retrieve variable names directly, using nameof
Detect next immediate attribute name, using will
Fetch argument names/sources passed to a function using argname
Read more from its documentation.
However, the final word I want to say is that, try to avoid using it whenever you can.
Because you can't make sure that the client code will run in an environment where the source node is available or AST node is accessible. And of course, it costs resources to parse the source code, identify the environment, retrieve the AST nodes and evaluate them when needed.
On python3, this function will get the outer most name in the stack:
import inspect
def retrieve_name(var):
"""
Gets the name of var. Does it from the out most frame inner-wards.
:param var: variable to get name from.
:return: string
"""
for fi in reversed(inspect.stack()):
names = [var_name for var_name, var_val in fi.frame.f_locals.items() if var_val is var]
if len(names) > 0:
return names[0]
It is useful anywhere on the code. Traverses the reversed stack looking for the first match.
I don't believe this is possible. Consider the following example:
>>> a = []
>>> b = a
>>> id(a)
140031712435664
>>> id(b)
140031712435664
The a and b point to the same object, but the object can't know what variables point to it.
def name(**variables):
return [x for x in variables]
It's used like this:
name(variable=variable)
>> my_var = 5
>> my_var_name = [ k for k,v in locals().items() if v == my_var][0]
>> my_var_name
'my_var'
In case you get an error if myvar points to another variable, try this (suggested by #mherzog)-
>> my_var = 5
>> my_var_name = [ k for k,v in locals().items() if v is my_var][0]
>> my_var_name
'my_var'
locals() - Return a dictionary containing the current scope's local variables.
by iterating through this dictionary we can check the key which has a value equal to the defined variable, just extracting the key will give us the text of variable in string format.
from (after a bit changes)
https://www.tutorialspoint.com/How-to-get-a-variable-name-as-a-string-in-Python
I wrote the package sorcery to do this kind of magic robustly. You can write:
from sorcery import dict_of
columns = dict_of(n_jobs, users, queues, priorities)
and pass that to the dataframe constructor. It's equivalent to:
columns = dict(n_jobs=n_jobs, users=users, queues=queues, priorities=priorities)
Here's one approach. I wouldn't recommend this for anything important, because it'll be quite brittle. But it can be done.
Create a function that uses the inspect module to find the source code that called it. Then you can parse the source code to identify the variable names that you want to retrieve. For example, here's a function called autodict that takes a list of variables and returns a dictionary mapping variable names to their values. E.g.:
x = 'foo'
y = 'bar'
d = autodict(x, y)
print d
Would give:
{'x': 'foo', 'y': 'bar'}
Inspecting the source code itself is better than searching through the locals() or globals() because the latter approach doesn't tell you which of the variables are the ones you want.
At any rate, here's the code:
def autodict(*args):
get_rid_of = ['autodict(', ',', ')', '\n']
calling_code = inspect.getouterframes(inspect.currentframe())[1][4][0]
calling_code = calling_code[calling_code.index('autodict'):]
for garbage in get_rid_of:
calling_code = calling_code.replace(garbage, '')
var_names, var_values = calling_code.split(), args
dyn_dict = {var_name: var_value for var_name, var_value in
zip(var_names, var_values)}
return dyn_dict
The action happens in the line with inspect.getouterframes, which returns the string within the code that called autodict.
The obvious downside to this sort of magic is that it makes assumptions about how the source code is structured. And of course, it won't work at all if it's run inside the interpreter.
This function will print variable name with its value:
import inspect
def print_this(var):
callers_local_vars = inspect.currentframe().f_back.f_locals.items()
print(str([k for k, v in callers_local_vars if v is var][0])+': '+str(var))
***Input & Function call:***
my_var = 10
print_this(my_var)
***Output**:*
my_var: 10
>>> locals()['foo']
{}
>>> globals()['foo']
{}
If you wanted to write your own function, it could be done such that you could check for a variable defined in locals then check globals. If nothing is found you could compare on id() to see if the variable points to the same location in memory.
If your variable is in a class, you could use className.dict.keys() or vars(self) to see if your variable has been defined.
I have a method, and while not the most efficient...it works! (and it doesn't involve any fancy modules).
Basically it compares your Variable's ID to globals() Variables' IDs, then returns the match's name.
def getVariableName(variable, globalVariables=globals().copy()):
""" Get Variable Name as String by comparing its ID to globals() Variables' IDs
args:
variable(var): Variable to find name for (Obviously this variable has to exist)
kwargs:
globalVariables(dict): Copy of the globals() dict (Adding to Kwargs allows this function to work properly when imported from another .py)
"""
for globalVariable in globalVariables:
if id(variable) == id(globalVariables[globalVariable]): # If our Variable's ID matches this Global Variable's ID...
return globalVariable # Return its name from the Globals() dict
In Python, the def and class keywords will bind a specific name to the object they define (function or class). Similarly, modules are given a name by virtue of being called something specific in the filesystem. In all three cases, there's an obvious way to assign a "canonical" name to the object in question.
However, for other kinds of objects, such a canonical name may simply not exist. For example, consider the elements of a list. The elements in the list are not individually named, and it is entirely possible that the only way to refer to them in a program is by using list indices on the containing list. If such a list of objects was passed into your function, you could not possibly assign meaningful identifiers to the values.
Python doesn't save the name on the left hand side of an assignment into the assigned object because:
It would require figuring out which name was "canonical" among multiple conflicting objects,
It would make no sense for objects which are never assigned to an explicit variable name,
It would be extremely inefficient,
Literally no other language in existence does that.
So, for example, functions defined using lambda will always have the "name" <lambda>, rather than a specific function name.
The best approach would be simply to ask the caller to pass in an (optional) list of names. If typing the '...','...' is too cumbersome, you could accept e.g. a single string containing a comma-separated list of names (like namedtuple does).
I think it's so difficult to do this in Python because of the simple fact that you never will not know the name of the variable you're using. So, in his example, you could do:
Instead of:
list_of_dicts = [n_jobs, users, queues, priorities]
dict_of_dicts = {"n_jobs" : n_jobs, "users" : users, "queues" : queues, "priorities" : priorities}
Many of the answers return just one variable name. But that won't work well if more than one variable have the same value. Here's a variation of Amr Sharaki's answer which returns multiple results if more variables have the same value.
def getVariableNames(variable):
results = []
globalVariables=globals().copy()
for globalVariable in globalVariables:
if id(variable) == id(globalVariables[globalVariable]):
results.append(globalVariable)
return results
a = 1
b = 1
getVariableNames(a)
# ['a', 'b']
just another way to do this based on the content of input variable:
(it returns the name of the first variable that matches to the input variable, otherwise None. One can modify it to get all variable names which are having the same content as input variable)
def retrieve_name(x, Vars=vars()):
for k in Vars:
if isinstance(x, type(Vars[k])):
if x is Vars[k]:
return k
return None
If the goal is to help you keep track of your variables, you can write a simple function that labels the variable and returns its value and type. For example, suppose i_f=3.01 and you round it to an integer called i_n to use in a code, and then need a string i_s that will go into a report.
def whatis(string, x):
print(string+' value=',repr(x),type(x))
return string+' value='+repr(x)+repr(type(x))
i_f=3.01
i_n=int(i_f)
i_s=str(i_n)
i_l=[i_f, i_n, i_s]
i_u=(i_f, i_n, i_s)
## make report that identifies all types
report='\n'+20*'#'+'\nThis is the report:\n'
report+= whatis('i_f ',i_f)+'\n'
report+=whatis('i_n ',i_n)+'\n'
report+=whatis('i_s ',i_s)+'\n'
report+=whatis('i_l ',i_l)+'\n'
report+=whatis('i_u ',i_u)+'\n'
print(report)
This prints to the window at each call for debugging purposes and also yields a string for the written report. The only downside is that you have to type the variable twice each time you call the function.
I am a Python newbie and found this very useful way to log my efforts as I program and try to cope with all the objects in Python. One flaw is that whatis() fails if it calls a function described outside the procedure where it is used. For example, int(i_f) was a valid function call only because the int function is known to Python. You could call whatis() using int(i_f**2), but if for some strange reason you choose to define a function called int_squared it must be declared inside the procedure where whatis() is used.
Maybe this could be useful:
def Retriever(bar):
return (list(globals().keys()))[list(map(lambda x: id(x), list(globals().values()))).index(id(bar))]
The function goes through the list of IDs of values from the global scope (the namespace could be edited), finds the index of the wanted/required var or function based on its ID, and then returns the name from the list of global names based on the acquired index.
Whenever I have to do it, mostly while communicating json schema and constants with the frontend I define a class as follows
class Param:
def __init__(self, name, value):
self.name = name
self.value = value
Then define the variable with name and value.
frame_folder_count = Param({'name':'frame_folder_count', 'value':10})
Now you can access the name and value using the object.
>>> frame_folder_count.name
'frame_folder_count'
>>> def varname(v, scope=None):
d = globals() if not scope else vars(scope); return [k for k in d if d[k] == v]
...
>>> d1 = {'a': 'ape'}; d2 = {'b': 'bear'}; d3 = {'c': 'cat'}
>>> ld = [d1, d2, d3]
>>> [varname(d) for d in ld]
[['d1'], ['d2'], ['d3']]
>>> d5 = d3
>>> [varname(d) for d in ld]
[['d1'], ['d2'], ['d3', 'd5']]
>>> def varname(v, scope=None):
d = globals() if not scope else vars(scope); return [k for k in d if d[k] is v]
...
>>> [varname(d) for d in ld]
[['d1'], ['d2'], ['d3', 'd5']]
As you see and is noted here, there can be multiple variables with the same value or even address, so using a wrapper to keep the names with the data is best.
Following method will not return the name of variable but using this method you can create data frame easily if variable is available in global scope.
class CustomDict(dict):
def __add__(self, other):
return CustomDict({**self, **other})
class GlobalBase(type):
def __getattr__(cls, key):
return CustomDict({key: globals()[key]})
def __getitem__(cls, keys):
return CustomDict({key: globals()[key] for key in keys})
class G(metaclass=GlobalBase):
pass
x, y, z = 0, 1, 2
print('method 1:', G['x', 'y', 'z']) # Outcome: method 1: {'x': 0, 'y': 1, 'z': 2}
print('method 2:', G.x + G.y + G.z) # Outcome: method 2: {'x': 0, 'y': 1, 'z': 2}
A = [0, 1]
B = [1, 2]
pd.DataFrame(G.A + G.B) # It will return a data frame with A and B columns
Some of the previous cases would fail if there are two variables with the same value. So it is convenient to alert it:
Defining function:
# Variable to string of variable name
def var_name(variable,i=0):
results = []
for name in globals():
if eval(name) == variable:
results.append(name)
if len(results) > 1:
print('Warning:' )
print(' var_name() has found',len(results), 'possible outcomes.')
print(' Please choose the suitable parameter "i". Where "i" is the index')
print(' that matches your choice from the list below.')
print(' ',results) ; print('')
return results[i]
Use:
var_1 = 10
var_name(var_1) # Output will be "var_1"
If you have 2 variables with the same value like var_1 = 8 and var_2 = 8, then a warning will appear.
var_1 = 8
var_2 = 8
var_name(var_2) # Output will be "var_1" too but Warning will appear
You can get your variable as kwargs and return it as string:
var=2
def getVarName(**kwargs):
return list(kwargs.keys())[0]
print (getVarName(var = var))
Note: variable name must be equal to itself.
I try to get name from inspect locals, but it cann't process var likes a[1], b.val.
After it, I got a new idea --- get var name from the code, and I try it succ!
code like below:
#direct get from called function code
def retrieve_name_ex(var):
stacks = inspect.stack()
try:
func = stacks[0].function
code = stacks[1].code_context[0]
s = code.index(func)
s = code.index("(", s + len(func)) + 1
e = code.index(")", s)
return code[s:e].strip()
except:
return ""
You can try the following to retrieve the name of a function you defined (does not work for built-in functions though):
import re
def retrieve_name(func):
return re.match("<function\s+(\w+)\s+at.*", str(func)).group(1)
def foo(x):
return x**2
print(retrieve_name(foo))
# foo
When finding the name of a variable from its value,
you may have several variables equal to the same value,
for example var1 = 'hello' and var2 = 'hello'.
My solution:
def find_var_name(val):
dict_list = []
global_dict = dict(globals())
for k, v in global_dict.items():
dict_list.append([k, v])
return [item[0] for item in dict_list if item[1] == val]
var1 = 'hello'
var2 = 'hello'
find_var_name('hello')
Outputs
['var1', 'var2']
Compressed version of iDilip's answer:
import inspect
def varname(x):
return [k for k,v in inspect.currentframe().f_back.f_locals.items() if v is x][0]
hi = 123
print(varname(hi))
It's totally possible to get the name of an instance variable, so long as it is the property of a class.
I got this from Effective Python by Brett Slatkin. Hope it helps someone:
The class must implement the get, set, and set_name dunder methods, which are part of the "Descriptor Protocol"
This worked when I ran it:
class FieldThatKnowsItsName():
def __init__(self):
self.name = None
self._value= None
self.owner = None
def __set_name__(self, owner, name):
self.name = name
self.owner = owner
self.owner.fields[self.name] = self
def __get__(self, instance, instance_type):
return self
def __set__(self, instance, value):
self = value
class SuperTable:
fields = {}
field_1=FieldThatKnowsItsName()
field_2=FieldThatKnowsItsName()
table = SuperTable()
print(table.field_1.name)
print(table.field_2.name)
You can then add methods and or extend your datatype as you like.
As a bonus, the set_name(self, owner, name) dunder also passes the parent instance, so the Field class instance can register itself with the parent.
I got this from Effective Python by Brett Slatkin. It took a while to figure out how to implement.
How can I do the same for a variable? As opposed to functions, Python variables do not have the __name__ attribute.
The problem comes up because you are confused about terminology, semantics or both.
"variables" don't belong in the same category as "functions". A "variable" is not a thing that takes up space in memory while the code is running. It is just a name that exists in your source code - so that when you're writing the code, you can explain which thing you're talking about. Python uses names in the source code to refer to (i.e., give a name to) values. (In many languages, a variable is more like a name for a specific location in memory where the value will be stored. But Python's names actually name the thing in question.)
In Python, a function is a value. (In some languages, this is not the case; although there are bytes of memory used to represent the actual executable code, it isn't a discrete chunk of memory that your program logic gets to interact with directly.) In Python, every value is an object, meaning that you can assign names to it freely, pass it as an argument, return it from a function, etc. (In many languages, this is not the case.) Objects in Python have attributes, which are the things you access using the . syntax. Functions in Python have a __name__ attribute, which is assigned when the function is created. Specifically, when a def statement is executed (in most languages, creation of a function works quite differently), the name that appears after def is used as a value for the __name__ attribute, and also, independently, as a variable name that will get the function object assigned to it.
But most objects don't have an attribute like that.
In other words, if I have a variable such as:
That's the thing: you don't "have" the variable in the sense that you're thinking of. You have the object that is named by that variable. Anything else depends on the information incidentally being stored in some other object - such as the locals() of the enclosing function. But it would be better to store the information yourself. Instead of relying on a variable name to carry information for you, explicitly build the mapping between the string name you want to use for the object, and the object itself.
So I have created a function that applies an action (in this case point wise multiplication of an array with a sinusoid, but that does not matter for my question) to an array.
Now I have created another function with which I want to create a string of python code to apply the first function multiple times later-on. The input of the second function can be either a string or an array, so that I can use the second function on its own output as well, if need be. My method of getting the variable name in a string works outside of the function.
Input :
var = np.array([[1,3],[2,4]]) # or sometimes var = 'a string'
if type(var)==str:
var_name = var
else:
var_name = [ k for k,v in locals().items() if v is var][0]
var_name
Output :
'var'
So here var is the variable (either array or string) supplied to the function, in this case an array. The if statement nicely returns me its name.
However when I use this inside my function, no matter what input I give it, it actually seems to look for var in locals(). Somehow it does not take var from the function input.
Definition :
def functionTWO(var, listoflistsofargs=None):
if type(var)==str:
var_name = var
else:
var_name = [ k for k,v in locals().items() if v is var][0]
if listoflistsofargs==None:
return var_name
command = []
for i in range(len(listoflistsofargs)):
if i==0:
command.append('functionONE(')
command.append(var_name)
command.append(',%.17f, %.17f)' % tuple(listoflistsofargs[i]))
else:
command.insert(0,'functionONE(')
command.append(',%.17f, %.17f)' % tuple(listoflistsofargs[i]))
''.join(command)
command[0] = var_name + ' + ' + command[0]
return ''.join(command)
Input :
somearray = np.array([[1,2,3],[1,2,3],[1,2,3]])
args = [[1,3],[6,5]]
command = functionTWO(somearray, args)
command
Output :
NameError: name 'var' is not defined
Wanted output :
'functionONE(functionONE(somearray, 1, 3), 6, 5)'
Why is listoflistsofargs taken from the function input and var not? I specify var in the listcomprehension in the definition of functionTWO. Normally when I use list comprehensions with function inputs it works fine. Does anybody know why this isnt the case here? Thank you in advance!
EDIT : So I guess the answer is dont. The implementation of classes by Marcin looks much cleaner and about the same order of amount of code. Too bad I couldnt get this to work inside a function. For other donts (actually other ideas) about using variable names as strings there is this question, where I got the above list comprehension for variable names.
You cannot pass a variable as a string*, and you should not do so.
If you want to pass a value between functions, the normal way is to pass it in as a parameter, and out as a return value.
If that is inconvenient, the usual solution is an object: define a class which carries both the shared variable, and methods which act on the variable.
If you need to create command objects, it is much better to do so in a structured way. For example, if you want to pass a function, and parameters, you can literally just pass the function object and the parameters in a tuple:
def foo():
return (functionONE,somearray,1,3)
command = foo()
command[0](*command[1:])
If you want to embed such commands within commands, you'll likely want to wrap that up with a class, so you can recursively evaluate the parameters. In fact, here's a little evaluator:
def evaluator(object):
def __init__(self,func=None,params=None):
self.func = func
self.params = params
def eval(self,alternativeparams=None):
if alternativeparams is not None:
params = alternativeparams
else:
params = self.params
if params is not None:
evaluatedparams = (item() if callable(item) else item for item in params)
else: evaluatedparams = None
if func is not None:
return self.func(*(evaluatedparams or ()))
else: return evaluatedparams
def __call__(self, *params):
return self.eval(params if params else None)
Although there are hacks by which you can pass references to local variables out of a function, these are not a great idea, because you end up creating your own half-baked object system which is more difficult to understand.
* This is because a variable has a name (which is a string) and a context, which maps names to strings. So, you need, at least to pass a tuple to truly pass a variable.
presenter's answer works in python 3
def get_variable_name(*variable):
return [ k for k,v in globals().items() if v is variable[0]][0]
example:
>> test = 'sample string'
>> get_variable_name(test)
'test'
>>
The only issue is it is a little messy.
The reason you get the error NameError: name 'var' is not defined when you wrap it all into a function is the following: locals().items() refers to the locally defined variables. as a matter of fact, the locally defined variables in your functions are only the variables defined inside the function and those passed as arguments to the function.
As the name says, locals().items() is local, and will consist of different variables depending on where it is called in your script.
On the contrary globals().items() consists of all the global variables, so try using it instead in your functions, it should do the trick.
For more info, look up global and local variables as well as the scope of variables in Python.
I do scientific programming, and often want to show users prompts and variable pairs, let them edit the variables, and then do the calulations with the new variables. I do this so often, that I wrote a wxPython class to move this code out of the main program. You set up a list for each variable with the type of the variable (string, float, int), the prompt, and the variable's current value. You then place all of these lists in one big list, and my utility creates a neatly formated wxPython panel with prompts and the current values which can be edited.
When I started, I only had a few variables, so I would write out each variable.
s='this is a string'; i=1; f=3.14
my_list=[ ['s','your string here',s], ['i','your int here',i], ['f','your float here'],]
input_panel = Input(my_list)
# the rest of the window is created, the input_panel is added to the window, the user is
# allowed to make choices, and control returns when the user hits the calculate button
s,i,f = input_panel.results() # the .results() function returns the values in a list
Now I want to use this routine for a lot of variables (10-30), and this approach is breaking down. I can create the input list to the function over multiple lines using the list.append() statements. When the code returns from the function, though, I get this huge list that needs to be unpacked into the right variables. This is difficult to manage, and it looks like it will be easy to get the input list and output list out of sync. And worse than that, it looks kludgy.
What is the best way to pass lots of variables to a function in Python with extra information so that they can be edited, and then get the variables back so that I can use them in the rest of the program?
If I could pass the variables by reference into the function, then users could change them or not, and I would use the values once the program returned from the function. I would only need to build the input list over multiple lines, and there wouldn't be any possiblity of the input list getting out of sync with the output list. But Python doesn't allow this.
Should I break the big lists into smaller lists that then get combined into big lists for passing into and out of the functions? Or does this just add more places to make errors?
The simplest thing to do would be to create a class. Instead of dealing with a list of variables, the class will have attributes. Then you just use a single instance of the class.
There are two decent options that come to mind.
The first is to use a dictionary to gather all the variables in one place:
d = {}
d['var1'] = [1,2,3]
d['var2'] = 'asdf'
foo(d)
The second is to use a class to bundle all the arguments. This could be something as simple as:
class Foo(object):
pass
f = Foo()
f.var1 = [1,2,3]
f.var2 = 'asdf'
foo(f)
In this case I would prefer the class over the dictionary, simply because you could eventually provide a definition for the class to make its use clearer or to provide methods that handle some of the packing and unpacking work.
To me, the ideal solution is to use a class like this:
>>> class Vars(object):
... def __init__(self, **argd):
... self.__dict__.update(argd)
...
>>> x = Vars(x=1, y=2)
>>> x.x
1
>>> x.y
2
You can also build a dictionary and pass it like this:
>>> some_dict = {'x' : 1, 'y' : 2}
>>> #the two stars below mean to pass the dict as keyword arguments
>>> x = Vars(**some_dict)
>>> x.x
1
>>> x.y
2
You may then get data or alter it as need be when passing it to a function:
>>> def foo(some_vars):
... some_vars.z = 3 #note that we're creating the member z
...
>>> foo(x)
>>> x.z
3
If I could pass the variables by reference into the function, then users could change them or not, and I would use the values once the program returned from the function.
You can obtain much the same effect as "pass by reference" by passing a dict (or for syntactic convenience a Bunch, see http://code.activestate.com/recipes/52308/).
if you have a finite set of these cases, you could write specific wrapper functions for each one. Each wrapper would do the work of building and unpacking lists taht are passed to the internal function.
I would recommend using a dictionary
or a class to accumulate all details
about your variables
value
prompt text
A list to store the order in which you want them to be displayed
Then use good old iteration to prepare input and collect output
This way you will only be modifying a small manageable section of the code time and again.
Of course you should encapsulate all this into a class if your comfortable working with classes.
"""Store all variables
"""
vars = {}
"""Store the order of display
"""
order = []
"""Define a function that will store details and order of the variable definitions
"""
def makeVar(parent, order, name, value, prompt):
parent[name] = dict(zip(('value', 'prompt'), (value, prompt)))
order.append(name)
"""Create your variable definitions in order
"""
makeVar(vars, order, 's', 'this is a string', 'your string here')
makeVar(vars, order, 'i', 1, 'your int here')
makeVar(vars, order, 'f', 3.14, 'your float here')
"""Use a list comprehension to prepare your input
"""
my_list = [[name, vars[name]['prompt'], vars[name]['value']] for name in order]
input_panel = Input(my_list)
out_list = input_panel.results();
"""Collect your output
"""
for i in range(0, len(order)):
vars[order[i]]['value'] = out_list[i];