Develop Reference

How can I get the __doc__ string of the calling function [duplicate]

I want to write a function which returns the calling function:
def foo():
return get_calling_function() #should return 'foo' function object
There's numerous examples online how to get the calling function's name, but not how to get the actual object. I've come up with the following solution which gets the name, then looks it up in the calling function's global namespace. However this doesn't work for class functions since there you need the class name as well, and I image there's a bunch of other edge cases as well.
from inspect import stack
def get_calling_function():
return stack()[2][0].f_globals[stack()[1][3]]
So any advice how or if its possible to write this function so that it works generically (on Python 3, btw)? Thanks.

Calling can happen from any code object (and from an extension module/builtin): from exec, execfile, from module name space (during import), from within a class definition, from within a method / classmethod / staticmethod, from a decorated function/method, from within a nested function, ... - so there is no "calling function" in general, and the difficulty to do anything good with that.
The stack frames and their code objects are the most general you can get - and examine the attributes.
This one finds the calling function in many cases:
import sys, inspect
def get_calling_function():
"""finds the calling function in many decent cases."""
fr = sys._getframe(1) # inspect.stack()[1][0]
co = fr.f_code
for get in (
lambda:fr.f_globals[co.co_name],
lambda:getattr(fr.f_locals['self'], co.co_name),
lambda:getattr(fr.f_locals['cls'], co.co_name),
lambda:fr.f_back.f_locals[co.co_name], # nested
lambda:fr.f_back.f_locals['func'], # decorators
lambda:fr.f_back.f_locals['meth'],
lambda:fr.f_back.f_locals['f'],
):
try:
func = get()
except (KeyError, AttributeError):
pass
else:
if func.__code__ == co:
return func
raise AttributeError("func not found")
# Usage
def f():
def nested_func():
print get_calling_function()
print get_calling_function()
nested_func()
class Y:
def meth(self, a, b=10, c=11):
print get_calling_function()
class Z:
def methz(self):
print get_calling_function()
z = Z()
z.methz()
return z
#classmethod
def clsmeth(cls):
print get_calling_function()
#staticmethod
def staticmeth():
print get_calling_function()
f()
y = Y()
z = y.meth(7)
z.methz()
y.clsmeth()
##y.staticmeth() # would fail
It finds:
<function f at 0x012E5670>
<function nested_func at 0x012E51F0>
<bound method Y.meth of <__main__.Y instance at 0x01E41580>>
<bound method Z.methz of <__main__.Z instance at 0x01E63EE0>>
<bound method Z.methz of <__main__.Z instance at 0x01E63EE0>>
<bound method classobj.clsmeth of <class __main__.Y at 0x01F3CF10>>
However, it may fail to find the function, or find the wrong function, for example, finding the wrong decorator-generated wrapper when the same decorator is used on multiple functions with the same name in different scopes.

How to use a function outside a class as a property inside a class?

I'm having some problems. How we can define a function outside of a function that can be used in a class property? Also, how we can insert the self parameter into the function signature? I would like to visualize it like this:
>>> def a(self, x): #I thought maybe class will give "self" to this property function
... print(self)
...
>>> class aa:
... def __init__(self):
... pass
... #a
... def p():
... print('in it')
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in aa
TypeError: a() missing 1 required positional argument: 'x'
I want to define a function outside but to use inside of a class. Like a class's method as a property. How can I do this?

It's not really clear what you want your out-of-class function to do. There are a bunch of possibilities, but you may not know the terminology yet to describe it to us.
Here's the three I think are most likely:
You may want your function to be a decorator. That means you can apply it to a method with #decorator syntax to other functions, including methods in a class.
For this to work, your function needs to be written to accept a function object as its only argument. Whatever it returns is what will replace the function or method it was being called on, so usually you want to return a callable, but you could instead return a descriptor like property does. Try something like this:
def decorator(func):
def wrapper(self, *args, **kwargs):
print("in the wrapper")
result = func(self, *args, **kwargs)
print("wrapper is done")
return result
return wrapper
class Foo:
#decorator
def foo(self, x):
print("in foo(), x is", x)
f = Foo()
f.foo(1) # prints three messages
When you call the foo method, you're actually going to be calling the wrapper method that the decorator returned after it was applied to the original method (func). Because of how we wrote the wrapper, it will call func so the original method prints out its message too.
You may want to use property (a descriptor type) to call your out-of-class function. This is a less common way of using property than applying it as a decorator on a method, but it's not impossible. You could even have two different functions, one to be called when requesting the attribute, the other than will be called when setting it (but I'll demonstrate with just the getter):
def getter(obj):
print("in the getter")
return 1
class Foo2:
foo = property(getter)
f2 = Foo2()
print(f2.foo) # prints a message from the getter function first, then prints 1
Note that you can't use #decorator syntax when building a property this way. That is only legal syntax immediately before a function definition, and we're not defining any functions that way inside our class.
You may just want to copy a function defined outside of the class into it, without any decorator or property nonsense. This is the easiest one to do, it's just a simple assignment:
def func(self, x):
print("x is", x)
class Foo3:
method = func # just assign the global to a name in the class body
func = func # you can even use the same name if you don't mind confusing people
f3 = Foo3()
f3.method(1)
f3.func(2)

If you want to create a property that uses a function defined outside your class, it would be something like this:
def myfunc(self):
return self._p
class Foo:
def __init__(self, p):
self._p = p
p = property(myfunc)
f = Foo("Alpha")
f.p # gives "Alpha"
property accepts a function as its (first) argument. The function should have self as a parameter, and should return the value that you want the property to evaluate to.

Is it possible to pass a #property to a predefined function such that the property's underlying method is invoked when the function uses it?

I'm curious about the #property annotation in Python 3.
I know I can manipulate state between calls to a property like this ...
class Obj:
_x = 0
#property
def x(self):
self._x += 1
return self._x
obj = Obj()
print(obj.x)
print(obj.x)
... which prints:
1
2
However, is it possible keep this mechanism after it has been passed to a function?
Take the following function:
def f(x):
print(x)
print(x)
Say we cannot change it, meaning we can't simply pass the object to the function and use the properties directly. Is it possible to pass it only the property, or something to that effect, such that each time x is "used" in the function x increments? Is there maybe a way to do this by manipulating and passing the class or object itself? Something similar to __call__(self, ...), or adding #property to the class itself or the __init__(self, ...) of the class?
I've tried the following naive approaches ...
obj = Obj()
f(obj.x)
... and ...
obj = Obj()
f(getattr(obj, 'x'))
Both print ...
1
1
... which makes sense seeing as it is evaluated before it is passed to f. I also tried to extend Obj:
class Obj:
_x = 0
#property
def x(self):
self._x += 1
return self._x
def y(self):
return x
... and then go ...
obj = Obj()
f(obj.y)
... but it produces ...
<bound method Obj.y of <__main__.Obj object at 0x000002379E015748>>
<bound method Obj.y of <__main__.Obj object at 0x000002379E015748>>
This also makes sense seeing as we're simply getting the method itself and passing it to the f function.
Just a note: This isn't a real world problem/example. I'm simply trying to understand the principles and limits of python itself.

The property() wrapper is a descriptor, meaning that it is only invoked by dotted access, like a.x, and not by plain variable access.
To trigger a method invocation or function call with just a variable reference, I think it would be necessary to exec() code in a custom locals namespace:
>>> def add_ten(x):
return x + 10
>>> class NS(dict):
def __getitem__(self, key):
value = dict.__getitem__(self, key)
if key == 'x':
return add_ten(value)
return value
>>> exec('print(x + 2)', globals(), NS(x=5))
17
In the above example, just a reference to the variable x is enough to trigger a call to add_ten().
This is likely more trouble than its worth, but as you say, you just wanted to see what is possible :-)

__getattr__ returning lambda function requiring one argument does not work

I am in the process of learning Python 3 and just ran into the getattr function. From what I can tell, it is invoked when the attribute call is not found in the class definition as a function or a variable.
In order to understand the behaviour, I wrote the following test class (based on what I've read):
class Test(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
def __getattr__(self, itm):
if itm is 'test':
return lambda x: "%s%s" % (x.foo, x.bar)
raise AttributeError(itm)
And I then initate my object and call the non-existent function test which, expectedly, returns the reference to the function:
t = Test("Foo", "Bar")
print(t.test)
<function Test.__getattr__.<locals>.<lambda> at 0x01A138E8>
However, if I call the function, the result is not the expected "FooBar", but an error:
print(t.test())
TypeError: <lambda>() missing 1 required positional argument: 'x'
In order to get my expected results, I need to call the function with the same object as the first parameter, like this:
print(t.test(t))
FooBar
I find this behaviour rather strange, as when calling p.some_function(), is said to add p as the first argument.
I would be grateful if someone could shine some light over this headache of mine. I am using PyDev in Eclipse.

__getattr__ return values are "raw", they don't behave like class attributes, invoking the descriptor protocol that plain methods involve that causes the creation of bound methods (where self is passed implicitly). To bind the function as a method, you need to perform the binding manually:
import types
...
def __getattr__(self, itm):
if itm is 'test': # Note: This should really be == 'test', not is 'test'
# Explicitly bind function to self
return types.MethodType(lambda x: "%s%s" % (x.foo, x.bar), self)
raise AttributeError(itm)
types.MethodType is poorly documented (the interactive help is more helpful), but basically, you pass it a user-defined function and an instance of a class and it returns a bound method that, when called, implicitly passes that instance as the first positional argument (the self argument).
Note that in your specific case, you could just rely on closure scope to make a zero-argument function continue to work:
def __getattr__(self, itm):
if itm is 'test': # Note: This should really be == 'test', not is 'test'
# No binding, but referring to self captures it in closure scope
return lambda: "%s%s" % (self.foo, self.bar)
raise AttributeError(itm)
Now it's not a bound method at all, just a function that happens to have captured self from the scope in which it was defined (the __getattr__ call). Which solution is best depends on your needs; creating a bound method is slightly slower, but gets a true bound method, while relying on closure scope is (trivially, ~10ns out of >400ns) faster, but returns a plain function (which may be a problem if, for example, it's passed as a callback to code that assumes it's a bound method and can have __self__ and __func__ extracted separately for instance).

To get what you want, you need a lambda that doesn't take arguments:
return lambda: "%s%s" % (self.foo, self.bar)
But you should really use a property for this, instead.
class Test(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
#property
def test(self):
return "{}{}".format(self.foo, self.bar)
t = Test("Foo", "Bar")
print(t.test)
# FooBar
Note the lack of parentheses.
If you're absolutely determined that it must be a function, do this:
class Test(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
#property
def test(self):
return lambda: "{}{}".format(self.foo, self.bar)
t = Test("Foo", "Bar")
print(t.test())
# FooBar

You need to create something that behaves like a bound method, you could simply use functools.partial to bind the instance to the function:
from functools import partial
class Test(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
def __getattr__(self, itm):
if itm == 'test': # you shouldn't use "is" for comparisons!
return partial(lambda x: "%s%s" % (x.foo, x.bar), self)
raise AttributeError(itm)
The test:
t = Test("Foo", "Bar")
print(t.test)
# functools.partial(<function Test.__getattr__.<locals>.<lambda> at 0x0000020C70CA6510>, <__main__.Test object at 0x0000020C7217F8D0>)
print(t.test())
# FooBar

"I find this behaviour rather strange, as when calling
p.some_function(), is said to add p as the first argument."
some_function is actually a method, which is why it gets passed an instance implicitly when the method is "bound to an object." But plain functions don't work that way, only functions defined in the class body have this magic applied to them automagically. And actually, unbound methods (accessed via the class directly) function the same as normal functions! The terminology "bound and unbound" methods no longer applies, because in Python 3 we only have methods and functions (getting rid of the distinction between unbound methods and plain functions). When an instance is instantiated, accessing the attribute returns a method which implicitly calls the instance on invocation.
>>> class A:
... def method(self, x):
... return x
...
>>> a.method
<bound method A.method of <__main__.A object at 0x101a5b3c8>>
>>> type(a.method)
<class 'method'>
However, if you access the attribute of the class you'll see it's just a function:
>>> A.method
<function A.method at 0x101a64950>
>>> type(A.method)
<class 'function'>
>>> a = A()
Now, observe:
>>> bound = a.method
>>> bound(42)
42
>>> unbound = A.method
>>> unbound(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: method() missing 1 required positional argument: 'x'
But this is the magic of classes. Note, you can even add functions to classes dynamically, and they get magically turned into methods when you invoke them on an instance:
>>> A.method2 = lambda self, x: x*2
>>> a2 = A()
>>> a2.method2(4)
8
And, as one would hope, the behavior still applies to objects already created!
>>> a.method2(2)
4
Note, this doesn't work if you dynamically add to an instance:
>>> a.method3 = lambda self, x: x*3
>>> a.method3(3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: <lambda>() missing 1 required positional argument: 'x'
You have to do the magic yourself:
>>> from types import MethodType
>>> a.method4 = MethodType((lambda self, x: x*4), a)
>>> a.method4(4)
16
>>>

Notice that if you do print(t.__getattr__) you get something like <bound method Test.__getattr__ of <__main__.Test object at 0x00000123FBAE4DA0>>. The key point is that methods defined on an object are said to be 'bound' and so always take the object as the first parameter. Your lambda function is just an anonymous function not 'bound' to anything, so for it to access the object it needs to be explicitly passed in.
I presume you are only doing this to experiment with using `__getattr__', as what you are doing could be much more easily achieved by making your lambda a method on the object.

Python how to get the calling function (not just its name)?

I want to write a function which returns the calling function:
def foo():
return get_calling_function() #should return 'foo' function object
There's numerous examples online how to get the calling function's name, but not how to get the actual object. I've come up with the following solution which gets the name, then looks it up in the calling function's global namespace. However this doesn't work for class functions since there you need the class name as well, and I image there's a bunch of other edge cases as well.
from inspect import stack
def get_calling_function():
return stack()[2][0].f_globals[stack()[1][3]]
So any advice how or if its possible to write this function so that it works generically (on Python 3, btw)? Thanks.

Calling can happen from any code object (and from an extension module/builtin): from exec, execfile, from module name space (during import), from within a class definition, from within a method / classmethod / staticmethod, from a decorated function/method, from within a nested function, ... - so there is no "calling function" in general, and the difficulty to do anything good with that.
The stack frames and their code objects are the most general you can get - and examine the attributes.
This one finds the calling function in many cases:
import sys, inspect
def get_calling_function():
"""finds the calling function in many decent cases."""
fr = sys._getframe(1) # inspect.stack()[1][0]
co = fr.f_code
for get in (
lambda:fr.f_globals[co.co_name],
lambda:getattr(fr.f_locals['self'], co.co_name),
lambda:getattr(fr.f_locals['cls'], co.co_name),
lambda:fr.f_back.f_locals[co.co_name], # nested
lambda:fr.f_back.f_locals['func'], # decorators
lambda:fr.f_back.f_locals['meth'],
lambda:fr.f_back.f_locals['f'],
):
try:
func = get()
except (KeyError, AttributeError):
pass
else:
if func.__code__ == co:
return func
raise AttributeError("func not found")
# Usage
def f():
def nested_func():
print get_calling_function()
print get_calling_function()
nested_func()
class Y:
def meth(self, a, b=10, c=11):
print get_calling_function()
class Z:
def methz(self):
print get_calling_function()
z = Z()
z.methz()
return z
#classmethod
def clsmeth(cls):
print get_calling_function()
#staticmethod
def staticmeth():
print get_calling_function()
f()
y = Y()
z = y.meth(7)
z.methz()
y.clsmeth()
##y.staticmeth() # would fail
It finds:
<function f at 0x012E5670>
<function nested_func at 0x012E51F0>
<bound method Y.meth of <__main__.Y instance at 0x01E41580>>
<bound method Z.methz of <__main__.Z instance at 0x01E63EE0>>
<bound method Z.methz of <__main__.Z instance at 0x01E63EE0>>
<bound method classobj.clsmeth of <class __main__.Y at 0x01F3CF10>>
However, it may fail to find the function, or find the wrong function, for example, finding the wrong decorator-generated wrapper when the same decorator is used on multiple functions with the same name in different scopes.