weird behavior while using ternary operator in for loop in python 3.5 [duplicate]

Question

While I was testing ternary operator in python 3, I came across this weird phenomenon

import string
test = ""
testing = chr(0) if chr(0) in test else ""
for c in string.ascii_letters + testing:
    print(c)

would print a~Z 1 character per line, but

import string

test = ""
for c in string.ascii_letters + chr(0) if chr(0) in test else "":
    print(c)

would print nothing.
Can somebody give me an explanation?

Change to for c in string.ascii_letters + (chr(0) if chr(0) in test else "") and it will work. — Maroun
– Maroun, Commented May 4, 2017 at 8:22
Are there cases or platforms where a Python string contains a NUL character? — user707650
– user707650, Commented May 4, 2017 at 8:23
operator precedence. When you have a doubt, use parentheses. — Jean-François Fabre
– Jean-François Fabre ♦, Commented May 4, 2017 at 8:23
why is the variable test or the if statement being used? as just doing this for c in string.ascii_letters: print(c) gives the same functionality — WhatsThePoint
– WhatsThePoint, Commented May 4, 2017 at 8:27

Daniel Roseman · Accepted Answer · 2017-05-04 08:25:02Z

0

This is due to operator precedence: + binds tighter than if.

In the first snippet, testing evalutes to "" because chr(0) is not in test. So, the loop is over ascii_letters + "", ie just the letters.

In the second, the + is evaluated first; the if therefore evaluates the whole thing to "" and the loop is just over the empty string.

answered May 4, 2017 at 8:25

Daniel Roseman

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Comments

Maroun · Accepted Answer · 2017-05-04 08:23:51Z

0

Change to:

for c in string.ascii_letters + (chr(0) if chr(0) in test else ""):

and it will work.

Why? Operator precedence. Your current code is actually:

for c in (string.ascii_letters + chr(0)) if chr(0) in test else "":

answered May 4, 2017 at 8:23

Maroun

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