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

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.

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.

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