Python order precedence (in vs "==")

Question

Anyone have a description as to why this is giving behavior?

if (key in self.dict) == True:

vs

if key in self.dict == True:

My order of precedence table tells me these two operators should evaluate left to right, but for some reason I'm actually seeing incorrect behavior in the second example I gave. Does anyone know why the parenthesis are important here?

https://docs.python.org/3/reference/expressions.html#operator-precedence

From my understanding these two lines should evaluate exactly the same.

Reproducable:

Python 3.8.3 (tags/v3.8.3:6f8c832, May 13 2020, 22:20:19) [MSC v.1925 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> dict = {}
>>> dict[1] = 2
>>> 1 in dict
True
>>> 1 in dict == True
False
>>> (1 in dict) == True
True
>>> 1 in (dict == True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument of type 'bool' is not iterable

Hmmm... `"a" in "a" == True` is `False`; `("a" in "a") == True` is `True`; but `"a" in ("a" == True)` gives a `TypeError`. — Mateen Ulhaq, Jun 15 '20 at 07:42
I agree, don't understand what's going on here. Will update with reproducible steps — blurb, Jun 15 '20 at 07:43
@MateenUlhaq Of course. You can't check if something is "in" False or True. — kabanus, Jun 15 '20 at 07:43
@kabanus Sure, the point is that "`==` is being evaluated first" (precedence) is not the right conclusion. — Mateen Ulhaq, Jun 15 '20 at 07:44

score 1 · Accepted Answer · edited Jun 15 '20 at 07:46

1

The relevant part is comparisons:

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z

So you have

key in dict and dict == True

This is very much a unique Python thing.

edited Jun 15 '20 at 07:46

Nick

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answered Jun 15 '20 at 07:45

kabanus

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Python order precedence (in vs "==")

1 Answers1