CODEWITHSUNDEEP
pythonpython-3.xconditional-statementsoperatorsboolean-expression
youngdev
youngdev

Reputation: 31

Certain conditions evaluate differently then expected in Python

Currently creating a piece that takes values a given value in an array and adds 32

My IDE (PyCharm) recommended that I remove a redundancy in my code by doing the following

if ascii_key[i] >= 65 and ascii_key[i] <= 90:
    ascii_key[i] = ascii_key[i] + 32

to

if 65 <= ascii_key[i] <= 90:
    ascii_key[i] = ascii_key[i] + 32

Both solutions work for me, but I'm curious as to why this arrangement of conditions breaks

if ascii_key[i] <= 65 >= 90:
    ascii_key[i] = ascii_key[i] + 32

I'd appreciate an explanation in pseudo-code if convenient, I'm still a bit novice

Upvotes: 0

Views: 42

Answers (2)

LetItBeAndre
LetItBeAndre

Reputation: 151

it is due to how the conditions are resolved. if ascii_key[i] >= 65 and ascii_key[i] <= 90 and if 65 <= ascii_key[i] <= 90 are equivalent. While if ascii_key[i] <= 65 >= 90 gets resolved to this: if ascii_key[i] <= 65 and 65 >= 90 65 >= 90 is never true.

First the condition 65 <= ascii_key[i] and then the other condition ascii_key[i] <= 90 are evaluated and chained with an AND.

Cheers

Upvotes: 0

Ch3steR
Ch3steR

Reputation: 20669

In the third code provided by you.

ascii_key[i] <= 65 >= 90 this is always evaluated to False.

ascii_key[i] <= 65 >= 90 is evaluated as ascii_key[i] <= 65 and 65 >= 90. 65>=90 is False.

Byte-code evaluation of a<b<c.

In [17]: dis.dis('a<b<c')
  1           0 LOAD_NAME                0 (a)
              2 LOAD_NAME                1 (b)
              4 DUP_TOP
              6 ROT_THREE
              8 COMPARE_OP               0 (<)
             10 JUMP_IF_FALSE_OR_POP    18
             12 LOAD_NAME                2 (c)
             14 COMPARE_OP               0 (<)
             16 RETURN_VALUE
        >>   18 ROT_TWO
             20 POP_TOP
             22 RETURN_VALUE

Byte-code evaluation of a<b and b<c.

In [18]: dis.dis('a<b and b<c')
  1           0 LOAD_NAME                0 (a)
              2 LOAD_NAME                1 (b)
              4 COMPARE_OP               0 (<)
              6 JUMP_IF_FALSE_OR_POP    14
              8 LOAD_NAME                1 (b)
             10 LOAD_NAME                2 (c)
             12 COMPARE_OP               0 (<)
        >>   14 RETURN_VALUE

You can observe observe a<b<c is equivalent to a<b and b<c.

Upvotes: 3

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