pylint should warn about type(x) == Y and type(x) is Y

[pylint-bot]

Originally reported by: Chris Rebert (BitBucket: cvrebert, GitHub: @cvrebert?)

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People who are new to Python often forget that the isinstance() built-in function exists, and instead end up writing their typechecks non-idiomatically, using type() and an equality comparison. Here's an example:

#!python

def dispatch(x):
    if type(x) == int:
        print "int"
    elif type(x) is dict:
        print "dict"

pylint doesn't currently emit any relevant warnings for this code. I propose it should emit warnings suggesting that the code be rewritten to use isinstance() instead.

Corrected example:

#!python

def typecheck(x):
    if isinstance(x, int):
        print "int"
    elif isinstance(x, dict):
        print "dict"

(Yes, I know that these aren't strictly equivalent due to subclasses and metaclasses, but those cases are pretty uncommon.)

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• Bitbucket: https://bitbucket.org/logilab/pylint/issue/299

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore):

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Merged in cvrebert/pylint/impl-299 (pull request #227)

Fix #299: Warn about type(x) is/== Y

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore):

---

Fix #299: Warn about type(x) is/== Y

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore):

---

Merged in cvrebert/pylint/impl-299 (pull request #227)

Fix #299: Warn about type(x) is/== Y