Operation of filter (None, list)

A memo when you are worried about the operation of the built-in function filter

I thought that filter is a high-rise function and takes a function as an argument, but apparently it can take None as an argument.

Anxious behavior

>>> a = [0,1,1.1,'a',None,False]
>>> b = filter(None,a)
>>> list(b)
[1, 1.1, 'a']

As for the operation, it seems that 0, None, False which are False in bool are removed.

>>> b = filter(lambda x:bool(x),a)
>>> list(b)
[1, 1.1, 'a']

Solution

Confirmed using help

help(filter)

class filter(object)
 |  filter(function or None, iterable) --> filter object
 |
 |  Return an iterator yielding those items of iterable for which function(item)
 |  is true. If function is None, return the items that are true.
 |
 |  Methods defined here:
 |
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |
 |  __iter__(self, /)
 |      Implement iter(self).
 |
 |  __next__(self, /)
 |      Implement next(self).
 |
 |  __reduce__(...)
 |      Return state information for pickling.
 |
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.

When None comes to the function part, it seems to behave like filter (lambda x: x, list).

bonus

When I tried to check it from the source code, it was quite difficult.

Checking the source using inspect cannot be done with the builtin function.

>>> import inspect
>>> inspect.getsource(filter)
...
TypeError: <class 'filter'> is a built-in class

I checked it from Documentation on Github, but the source is not included. That should be it, the built-in functions are written in C. You can check the source code of the filter function as of Python2 with builtin_filter in Mercurial. I wonder if this hasn't changed even after becoming Python3 ...?