Using ternary operator in python?

Question

Consider the following code snippet. It flags a syntax error at the break statement.

digits = list(str(102))
dummy = list(str(102/2))
for j in digits:
    dummy.remove(j) if j in dummy else break

How do I fix this?(I want to still use the ternary operator)

break is a statement, and as such it can't be used inside a ternary. Sorry. — Mark Ransom
– Mark Ransom, Commented May 13, 2016 at 15:50
To remove an element from a list you probably need to be careful (safer is to do the reverse way). check this out: stackoverflow.com/questions/35618307/… see also Padraic's answer — Ian
– Ian, Commented May 13, 2016 at 16:22
@MarkRansom That's wrong reasoning, as expressions are statements and they can be used inside a ternary. — Stefan Pochmann
– Stefan Pochmann, Commented May 13, 2016 at 16:24
@StefanPochmann: Expressions are statements - but not all statements are expressions (break being an example), and the ternary operator requires expressions (not statements). — Martin Bonner supports Monica
– Martin Bonner supports Monica, Commented May 13, 2016 at 16:29

Community · Accepted Answer · 2017-05-23 12:16:04Z

6

Edit:

(see my conversation with Stefan Pochmann in the comments)

Ternary operator is not for only statement, but rather for assignment or for expression (and break is an only statement):

a = 5 if cond else 3 #OK
do() if cond else dont() #also OK
do() if cond else break #not OK

use normal if-else statement to do statements:

if cond:
    do()
else:
    break

edited May 23, 2017 at 12:16

CommunityBot

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answered May 13, 2016 at 15:52

Ian

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16 Comments

jonrsharpe Over a year ago

Your examples aren't quite the same, as they are valid syntax

Martijn Pieters Over a year ago

@TheChetan: no, there isn't. Python focuses on readability, not conciseness.

Ian Over a year ago

@TheChetan hmm... depends on the case, you could in fact make for loop and store the functions in list first for instance

Ian Over a year ago

@sparkandshine ah I get it... that is because exit is a function. If you use tool like PyCharm you can see that.. it is like function returning nothing

Stefan Pochmann Over a year ago

@Ian Ah, ok. The problem is that they're only statements, not expressions, and you need an expression there.

|

Padraic Cunningham · Accepted Answer · 2016-05-13 16:45:04Z

3

You cannot use break in Your loop logic can be re written using itertools.takewhile if you want a more succinct solution

digits = list(str(102))
dummy = list(str(102/2))

from itertools import takewhile

for d in takewhile(dummy.__contains__, digits):
    dummy.remove(d)

You can also remove the need for the else using a for loop by reversing your logic, check if j is not in dummy breaking when that is True:

for j in digits:
    if j not in dummy:
        break
    dummy.remove(j)

Also if you want to remove all occurrences of any of the initial elements from digits that are in dummy, remove won't do that for any repeating elements but using a list comp after creating a set of elements to remove will:

digits = str(102)
dummy = list(str(102/2))
st = set(takewhile(dummy.__contains__, digits))
dummy[:] = [d for d in dummy if d not in st]

print(dummy)

You can also iterate over a string so no need to call list on digits unless you plan on doing some list operations with it after.

edited May 13, 2016 at 16:45

answered May 13, 2016 at 16:18

Padraic Cunningham

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2 Comments

Jared Goguen Over a year ago

Ugh, it's awkward to avoid the dunder method here; you can use functools.partial and operator.contains, e.g. in_dummy = partial(contains, dummy). This seems like a lot of work to avoid the underscore method though, and no real benefits. A lambda could be used alternatively, e.g. lambda e: e in dummy.

Padraic Cunningham Over a year ago

@JaredGoguen, yep, I don't mind using __contains__ in a case like this, definitely better than a lambda, might be interesting to time the comparisons.

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