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Is it possible to use 'else' in a python list comprehension?

Posted by: admin November 1, 2017 Leave a comment

Questions:

Here is the code I was trying to turn into a list comprehension:

table = ''
for index in xrange(256):
    if index in ords_to_keep:
        table += chr(index)
    else:
        table += replace_with

Is there a way to add the else statement to this comprehension?

table = ''.join(chr(index) for index in xrange(15) if index in ords_to_keep)
Answers:

The syntax a if b else c is a ternary operator in Python that evaluates to a if the condition b is true – otherwise, it evaluates to c. It can be used in comprehension statements:

>>> [a if a else 2 for a in [0,1,0,3]]
[2, 1, 2, 3]

So for your example,

table = ''.join(chr(index) if index in ords_to_keep else replace_with
                for index in xrange(15))

Questions:
Answers:

If you want an else you don’t want to filter the list comprehension, you want it to iterate over every value. You can use true-value if cond else false-value as the statement instead, and remove the filter from the end:

table = ''.join(chr(index) if index in ords_to_keep else replace_with for index in xrange(15))

Questions:
Answers:

To use the else in list comprehensions in python programming you can try out the below snippet. This would resolve your problem, the snippet is tested on python 2.7 and python 3.5.

obj = ["Even" if i%2==0 else "Odd" for i in range(10)]

Questions:
Answers:

Also, would I be right in concluding that a list comprehension is the most efficient way to do this?

Maybe. List comprehensions are not inherently computationally efficient. It is still running in linear time.

From my personal experience:
I have significantly reduced computation time when dealing with large data sets by replacing list comprehensions (specifically nested ones) with for-loop/list-appending type structures you have above. In this application I doubt you will notice a difference.

Questions:
Answers:

Great answers, but just wanted to mention a gotcha that “pass” keyword will not work in the if/else part of the list-comprehension (as posted in the examples mentioned above).

#works
list1 = [10, 20, 30, 40, 50]
newlist2 = [x if x > 30 else x**2 for x in list1 ]
print(newlist2, type(newlist2))

#but this WONT work
list1 = [10, 20, 30, 40, 50]
newlist2 = [x if x > 30 else pass for x in list1 ]
print(newlist2, type(newlist2))

This is tried and tested on python 3.4.
Error is as below:

newlist2 = [x if x > 30 else pass for x in list1 ]                                    
SyntaxError: invalid syntax

So, try to avoid pass-es in list comprehensions