How do I detect whether a variable is a function?

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 "", line 1, in ? NameError: name 'function' is not defined ``` The reason I picked that is because ``` >>> type(x) ```

Accepted 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.

How do I detect whether a variable is a function?

Solution

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