How does a list comprehension in python avoid side effects?

Asked Jul 05 '17 at 03:30

Active Jul 05 '17 at 03:44

Viewed 42 times

We can calculate the list of squares without any side effects using:

squares = [x**2 for x in range(10)]

My question is - How is this without side effects ?

the above code also creates (or overwrites) a variable named x that still exists after the loop completes.

Edited : since I've been asked for code samples (this is from python 2.7)

x = 0
print "before list comprehension"
print "x = "+str(x)
squares = [x**2 for x in range(10)]
print "after list comprehension"
print "x = "+str(x)

Output -

before list comprehension
x = 0
after list comprehension
x = 9

edited Jul 05 '17 at 03:44

Bryan Oakley

asked Jul 05 '17 at 03:30

Thalish Sajeed

1

Did you try printing `x` after that line of code to see what happens? – idjaw Jul 05 '17 at 03:32
Yes. the value of x changes depending on the argument given to range. In the above case - The value of x was 9 – Thalish Sajeed Jul 05 '17 at 03:33
please post your code where this happens. – PYA Jul 05 '17 at 03:34
Please post a complete example replicating your observation. The `x` variable exists only within the scope of that comprehension. – idjaw Jul 05 '17 at 03:35
1

In 2.7 it gives a value of 9 just as he describes. – systemjack Jul 05 '17 at 03:36
@systemjack can you post the code which gave you that example? – PYA Jul 05 '17 at 03:37
Please read the link I provided. That explains it. You have to be careful what version you are actually using based on the documentation you are reading. You were reading about Python 3 and using a Python 2 interpreter. – idjaw Jul 05 '17 at 03:40
@idjaw Thanks ! I should have been more careful. I've marked the question as a duplicate – Thalish Sajeed Jul 05 '17 at 03:41

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