[ Python 常見問題 ] How do I detect whether a Python variable is a function?

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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:

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1. >>> isinstance(x, function)  

But that gives me:

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

The reason I picked that is because

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1. >>> type(x)  
2. <type 'function'>  

HowTo
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:

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1. 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:

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1. hasattr(obj, '__call__')  

The oft-suggested types.FunctionType 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:

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1. >>> isinstance(open, types.FunctionType)  
2. False  
3. >>> callable(open)  
4. True  

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