CODEWITHSUNDEEP
pythontypescomparisonisinstance
Paolo
Paolo

Reputation: 21056

How to properly check object types in Python?

Problem: I have to check that the a returned value is a Python dictionary.

Q1. Which of these options is the proper way to do this?

type(x) == dict

type(x) == type(dict)

isinstance(d, dict)

Then there are the other variants using is operator instead of ==.

Q2. Many people say that checking the type of an object is generally a bad practice, but regarding to my initial problem, do I have any other choice?

Upvotes: 3

Views: 6494

Answers (3)

Karoly Horvath
Karoly Horvath

Reputation: 96258

Rely on behaviour, not on actual type (see other answers).

Lot of objects can act like dictionaries, you don't want to force users of your function/API to use plain dicts, right?

On the pragmatic side:

>>> type({})
<type 'dict'>
>>> dict
<type 'dict'>
>>> type(dict)
<type 'type'>
>>> isinstance({}, dict)
True
>>> isinstance("", dict)
False

Upvotes: 2

S.Lott
S.Lott

Reputation: 391820

Q1. Which of these options is the proper way to do this?

Don't waste time on type checking.

It's error-prone because it's based on assumptions.

Q2. ... do I have any other choice?

Yes do this.

try:
    x.the_dict_operation()
except TypeError:
    # x was not the expected type for the operation
    raise # or whatever.

In most cases, this amounts to "do nothing".

Just write the code. If "somehow" a malicious sociopath uses the wrong type, it will just crash like it's supposed to.

Upvotes: 8

Cat Plus Plus
Cat Plus Plus

Reputation: 129754

Check for __getitem__ rather than verifying the type. If you really want to type-check, any is fine (well, except type(x) == type(dict), that will never be true for a dict instance). isinstance is probably the most standard way to do it.

Upvotes: 1

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