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

Question

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)

Answer 1

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))

Answer 2

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))

Answer 3

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.