Determine the type of an object?

Question

Is there a simple way to determine if a variable is a list, dictionary, or something else? I am getting an object back that may be either type and I need to be able to tell

Answer 1

Determine the type of a Python object

Determine the type of an object with type

>>> obj = object()
>>> type(obj)

Although it works, avoid double underscore attributes like __class__ - they're not semantically public, and, while perhaps not in this case, the builtin functions usually have better behavior.

>>> obj.__class__ # avoid this!

type checking

Is there a simple way to determine if a variable is a list, dictionary, or something else? I am getting an object back that may be either type and I need to be able to tell the difference.

Well that's a different question, don't use type - use isinstance:

def foo(obj):
    """given a string with items separated by spaces, 
    or a list or tuple, 
    do something sensible
    """
    if isinstance(obj, str):
        obj = str.split()
    return _foo_handles_only_lists_or_tuples(obj)

This covers the case where your user might be doing something clever or sensible by subclassing str - according to the principle of Liskov Substitution, you want to be able to use subclass instances without breaking your code - and isinstance supports this.

Use Abstractions

Even better, you might look for a specific Abstract Base Class from collections or numbers:

from collections import Iterable
from numbers import Number

def bar(obj):
    """does something sensible with an iterable of numbers, 
    or just one number
    """
    if isinstance(obj, Number): # make it a 1-tuple
        obj = (obj,)
    if not isinstance(obj, Iterable):
        raise TypeError('obj must be either a number or iterable of numbers')
    return _bar_sensible_with_iterable(obj)

Or Just Don't explicitly Type-check

Or, perhaps best of all, use duck-typing, and don't explicitly type-check your code. Duck-typing supports Liskov Substitution with more elegance and less verbosity.

def baz(obj):
    """given an obj, a dict (or anything with an .items method) 
    do something sensible with each key-value pair
    """
    for key, value in obj.items():
        _baz_something_sensible(key, value)

Conclusion

• Use type to actually get an instance's class.
• Use isinstance to explicitly check for actual subclasses or registered abstractions.
• And just avoid type-checking where it makes sense.