Python replace values in list

Question

I often write code like this:

newarguments = []
for argument in arguments:
 if parser.is_label(argument):
 argument = str(labels[argument])
 newarguments.append(argument)

but this feels really unpythonic. Is there a better way to loop over a list and replace certain values?

• 200_success: This question lacks context. See How to Ask. What are the inputs and outputs? What is the parser? Please be specific. If, by "I often write code like this", you are asking for advice about unspecified hypothetical code, then this question is off-topic for Code Review.
• redfast00: @200_success I don't really think that matters in this case since I was asking how to loop over a list and replace values in a Pythonic way (this pattern often comes back in my code). If that makes it off-topic, I'll add some context tomorrow. It would be a shame to delete this question since alecxe wrote such a good answer. Should this be moved to Stack Overflow?

Answer 1

A common way to collapse a list "mapping" code block like this is to use a list comprehension combined with a ternary conditional operator:

newarguments = [str(labels[argument]) if parser.is_label(argument) else argument 
 for argument in arguments]

List comprehensions are also generally faster (reference).

• redfast00: I considered list comprehension before asking this question, but didn't know about the 'else', leading to a bug that removed all non-labels. The docs you linked also don't mention that an else can be used in a list comprehension. I love to learn these 'hidden' features in Python!
• alecxe: @redfast00 gotcha, added a note about the if/else shorthand expression for completeness sake. Thanks.