How to check if len is valid

You can test if the object is Sized:

import collections.abc

if isinstance(bar, collections.abc.Sized):

The isinstance() test is true if all abstract methods of Sized are implemented; in this case that's just __len__.

Personally, I'd just catch the exception instead:

try:
    foo(42)
except TypeError:
    pass  # oops, no length

You can do:

if hasattr(bar, '__len__'):
    pass

Alternatively, you can catch the TypeError.

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