In Python, is it better to use list comprehensions or for-each loops?

python coding-style foreach list-comprehension
froadie picture froadie · May 17, 2010 · Viewed 15.4k times · Source

Which of the following is better to use and why?

Method 1:

for k, v in os.environ.items():
       print "%s=%s" % (k, v)

Method 2:

print "\n".join(["%s=%s" % (k, v) 
   for k,v in os.environ.items()])

I tend to lead towards the first as more understandable, but that might just be because I'm new to Python and list comprehensions are still somewhat foreign to me. Is the second way considered more Pythonic? I'm assuming there's no performance difference, but I may be wrong. What would be the advantages and disadvantages of these 2 techniques?

(Code taken from Dive into Python)

Answer

Steven D. Majewski picture Steven D. Majewski · May 17, 2010

If the iteration is being done for its side effect ( as it is in your "print" example ), then a loop is clearer.

If the iteration is executed in order to build a composite value, then list comprehensions are usually more readable.

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