python if condition and "and"

Question

 >>> def foo(a):
        print "called the function"
        if(a==1):
            return 1
        else:
            return None
>>> a=1

>>> if(foo(a) != None and foo(a) ==1):
    print "asdf"

called the function
called the function
asdf

Hi. how can i avoid calling the function twice without using an extra variable.

you can save the result of foo(a) into a variable. Then check the variable value in if() clause. — Raptor
– Raptor, Commented Mar 14, 2013 at 11:25
Exactly, for the second condition to be True, the first has to be. Also note that brackets around the condition are considered bad form in Python. if(a==1): should just be if a==1:. — Gareth Latty
– Gareth Latty, Commented Mar 14, 2013 at 11:26

John La Rooy · Accepted Answer · 2013-03-14 11:26:15Z

12

You can chain the comparisons like this

if None != foo(a) == 1:

This works like

if (None != foo(a)) and (foo(a) == 1):

except that it only evaluates foo(a) once.

answered Mar 14, 2013 at 11:26

John La Rooy

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6 Comments

salezica Over a year ago

You can chain equality comparisons? Well, look at that!

Gareth Latty Over a year ago

While this is a good general answer, the first check is still redundant in the given case.

John La Rooy Over a year ago

@Lattyware, I assume the given case was just a poor example. NPE has that covered anyway

Gareth Latty Over a year ago

@gnibbler The OP's comment on NPE's answer says that 'my actual problem is similar to this'. And yeah, I'm just making the point that often logical wrangling can be avoided by simplifying the logic beforehand.

boltsfrombluesky Over a year ago

i gave poor example but was actually looking for this solution. i didn't knew we can chain that way.

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NPE · Accepted Answer · 2013-03-14 11:25:51Z

7

how can i avoid calling the function twice without using an extra variable.

Here, you can simply replace

if(foo(a) != None and foo(a) ==1):

with

if foo(a) == 1:

The foo(a) != None is redundant: if foo(a) == 1, it is guaranteed to not be None.

answered Mar 14, 2013 at 11:25

NPE

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6 Comments

jimifiki Over a year ago

Maybe the posted code is not the real code but just an example. if(foo(a) != None and foo(a) ==1): sounds as a better solution

Gareth Latty Over a year ago

@jimifiki If the function has side effects, that could cause problems. Besides, this is the best answer to the asked question. If the OP has over-simplified it, then he needs to ask a new question.

salezica Over a year ago

This is not answering the question. I agree with @jimifiki. And no it's not the best, see below

boltsfrombluesky Over a year ago

my bad i didn't knew that. this is the answer i need . my actual problem is similar to this. this works for me.

Gareth Latty Over a year ago

@uʍopǝpısdn It answers the question - this in no way deserves a -1, as it is correct. It might not be as general a solution, but it's the best solution for the given problem.

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Steve Mayne · Accepted Answer · 2013-03-14 11:26:32Z

1

The following statement

if foo(a) == 1:

will deal with both conditions.

answered Mar 14, 2013 at 11:26

Steve Mayne

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Comments

zds_cn · Accepted Answer · 2013-03-14 11:43:50Z

1

if foo(a)==1 ,then foo(a) will not be None,

so simplify your code to:

if foo(a):
    print('asdf')

answered Mar 14, 2013 at 11:43

zds_cn

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2 Comments

Steve Mayne Over a year ago

Assuming that he means "Any value that evaluates to true" is a bit dodgy, I think.

zds_cn Over a year ago

i don't think there is a difference between if foo(a) and if foo(a)== 1 ,here.

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