How do I detect whether a Python variable is a function? – Dev

The best answers to the question “How do I detect whether a Python variable is a function?” in the category Dev.

QUESTION:

I have a variable, x, and I want to know whether it is pointing to a function or not.

I had hoped I could do something like:

>>> isinstance(x, function)

But that gives me:

Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'function' is not defined

The reason I picked that is because

>>> type(x)
<type 'function'>

ANSWER:

Builtin types that don’t have constructors in the built-in namespace (e.g. functions, generators, methods) are in the types module. You can use types.FunctionType in an isinstance call:

>>> import types
>>> types.FunctionType
<class 'function'>

>>> def f(): pass

>>> isinstance(f, types.FunctionType)
True
>>> isinstance(lambda x : None, types.FunctionType)
True

Note that this uses a very specific notion of “function” that is usually not what you need. For example, it rejects zip (technically a class):

>>> type(zip), isinstance(zip, types.FunctionType)
(<class 'type'>, False)

open (built-in functions have a different type):

>>> type(open), isinstance(open, types.FunctionType)
(<class 'builtin_function_or_method'>, False)

and random.shuffle (technically a method of a hidden random.Random instance):

>>> type(random.shuffle), isinstance(random.shuffle, types.FunctionType)
(<class 'method'>, False)

If you’re doing something specific to types.FunctionType instances, like decompiling their bytecode or inspecting closure variables, use types.FunctionType, but if you just need an object to be callable like a function, use callable.

ANSWER:

If this is for Python 2.x or for Python 3.2+, you can use callable(). It used to be deprecated, but is now undeprecated, so you can use it again. You can read the discussion here: http://bugs.python.org/issue10518. You can do this with:

callable(obj)

If this is for Python 3.x but before 3.2, check if the object has a __call__ attribute. You can do this with:

hasattr(obj, '__call__')

The oft-suggested types.FunctionTypes or inspect.isfunction approach (both do the exact same thing) comes with a number of caveats. It returns False for non-Python functions. Most builtin functions, for example, are implemented in C and not Python, so they return False:

>>> isinstance(open, types.FunctionType)
False
>>> callable(open)
True

so types.FunctionType might give you surprising results. The proper way to check properties of duck-typed objects is to ask them if they quack, not to see if they fit in a duck-sized container.

ANSWER:

The accepted answer was at the time it was offered thought to be correct. As it
turns out, there is no substitute for callable(), which is back in Python
3.2: Specifically, callable() checks the tp_call field of the object being
tested. There is no plain Python equivalent. Most of the suggested tests are
correct most of the time:

>>> class Spam(object):
...     def __call__(self):
...         return 'OK'
>>> can_o_spam = Spam()


>>> can_o_spam()
'OK'
>>> callable(can_o_spam)
True
>>> hasattr(can_o_spam, '__call__')
True
>>> import collections
>>> isinstance(can_o_spam, collections.Callable)
True

We can throw a monkey-wrench into this by removing the __call__ from the
class. And just to keep things extra exciting, add a fake __call__ to the instance!

>>> del Spam.__call__
>>> can_o_spam.__call__ = lambda *args: 'OK?'

Notice this really isn’t callable:

>>> can_o_spam()
Traceback (most recent call last):
  ...
TypeError: 'Spam' object is not callable

callable() returns the correct result:

>>> callable(can_o_spam)
False

But hasattr is wrong:

>>> hasattr(can_o_spam, '__call__')
True

can_o_spam does have that attribute after all; it’s just not used when calling
the instance.

Even more subtle, isinstance() also gets this wrong:

>>> isinstance(can_o_spam, collections.Callable)
True

Because we used this check earlier and later deleted the method, abc.ABCMeta
caches the result. Arguably this is a bug in abc.ABCMeta. That said,
there’s really no possible way it could produce a more accurate result than
the result than by using callable() itself, since the typeobject->tp_call
slot method is not accessible in any other way.

Just use callable()

ANSWER:

Since Python 2.1 you can import isfunction from the inspect module.

>>> from inspect import isfunction
>>> def f(): pass
>>> isfunction(f)
True
>>> isfunction(lambda x: x)
True