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If I execute following line of code inside function I get an exception: UnboundLocalError: local variable 'x' referenced before assignment but using the same line of code outside of function runs without any error. Why is that?

result = [{'year': 1990, 'days_in_mon':'1,2'}, {'year':1991, 'days_in_mon':'2,3'}] def test_fn(li): return [int(i) for i in x['days_in_mon'].split(",") for x in li if x['year'] == 1991] test_fn(result) # Gives an error `UnboundLocalError: local variable 'x' referenced before assignment` [int(i) for i in x['days_in_mon'].split(",") for x in result if x['year'] == 1991] # No Error

update

Expected result : [2,3]

What am I trying to do here?

I am trying to pull out value of of 'days_in_mon' from certain dict in a list and parse it as a list of numbers.

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  • check this [int(i) for x in a if x['a'] == 2 for i in x['b'].split(",") ] Commented Oct 9, 2017 at 11:11
  • what are u doing with nested loop ? Commented Oct 9, 2017 at 11:12
  • @pdshah I've updated my question to be more clear at what am I trying to do and what am I trying to achieve. Commented Oct 9, 2017 at 11:23
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    both [int(i) for i in x['days_in_mon'].split(",") for x in result if x['year'] == 1991] and [int(i) for i in x['days_in_mon'].split(",") for x in li if x['year'] == 1991] are same and raising exception inside and outside of function. @Igle's answer is correct. You have wrong implementation of list comprehension. Commented Oct 9, 2017 at 13:40

2 Answers 2

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Turn around your loops in the list comprehension:

 result = [{'year': 1990, 'days_in_mon':'1,2'}, {'year':1991, 'days_in_mon':'2,3'}] def test_fn(li): return [int(i) for x in li if x['year'] == 1991 for i in x['days_in_mon'].split(",")] test_fn(result) # returns [2, 3]

This is needed, as the first loop is evaluated before the second. So x must be initialized before executing the second one.

The way you wrote it, x is not defined:

print [int(i) for i in x['days_in_mon'].split(",") for x in result if x['year'] == 1991] NameError: name 'x' is not defined
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what u have:

[int(i) for i in x['days_in_mon'].split(",") for x in result if x['year'] == 1991] Traceback (most recent call last): File "<pyshell#1>", line 1, in <module> [int(i) for i in x['days_in_mon'].split(",") for x in result if x['year'] == 1991] NameError: name 'x' is not defined

what to be : 

>>> [int(i) for x in result if x['year'] == 1991 for i in x['days_in_mon'].split(",") ] [2, 3]

you write the nested loop in wrong way

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