Difference between unbracketed v. bracketed 'for loop' in python?

Question

I am very confused on why the following two for-loop scenarios generate different outputs, starting from the same list of lines:

 print lines
   ['175.11\n', '176.39\t56.887\n', '178.17\n', '176.1\t51.679\n', '176.1\t51.679\n', '175.15\n', '176.91\t32.149\t30.344\n', '182.33\n', '173.04\n', '174.31\n']

SCENARIO #1: Bracketed for-loop

When I run the following:

lines = ["Total = "+line for line in lines]
print lines

lines becomes:

['Total = 175.11\n', 'Total = 176.39\t56.887\n', 'Total = 178.17\n', 'Total = 176.1\t51.679\n', 'Total = 176.1\t51.679\n', 'Total = 175.15\n', 'Total = 176.91\t32.149\t30.344\n', 'Total = 182.33\n', 'Total = 173.04\n', 'Total = 174.31\n']

SCENARIO #2: Un-bracketed for-loop

But, when I run this:

for line in lines:
    lines = ["Total = "+line]
print lines

lines becomes only:

['Total = 174.31\n']

I would greatly appreciate any help explaining what's going on here! (Also, I should mention that I am more interested in the output from SCENARIO #1, but would like to accomplish it using the format of SCENARIO #2).

Have you read [the Python tutorial](https://docs.python.org/tutorial)? What you are calling a "bracketed for loop" is in fact a list comprehension, which is not a for loop at all (although it has some similarities). — BrenBarn, Oct 10 '15 at 21:47
Ahh OK, thank you @BrenBarn! I didn't realize that...I didn't know how else to google "for loop in brackets". That explains a lot now. — Dennis, Oct 10 '15 at 21:49

score 3 · Answer 1 · edited May 23 '17 at 11:52

3

You are overwriting your list of each loop iteration, as opposed to appending to it.

The fix would be:

myList = []
for line in lines:
    myList.append("Total = " + line) # appends the r-value to your list

But I still prefer list comprehension for its conciseness, anyway.

You can also use condition list comprehension:

# excludes empty lines
myList = ["Total = "+line for line in lines if len(line) > 0]

You are modifying a list that you are iterating through

As your for loop progresses through your lines you are appending new items. Any time you modify a container you are iterating through the results can be damaging. Read this question.

edited May 23 '17 at 11:52

Community

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answered Oct 10 '15 at 21:48

Aidan Gomez

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Isn't `list` a builtin in Python? – Peter Wood Oct 10 '15 at 21:52
@PeterWood my mistake. Edited to reflect that. – Aidan Gomez Oct 10 '15 at 21:53
Yes, `list` is the name for the builtin list type. – BrenBarn Oct 10 '15 at 21:56
`list` is indeed a built-in, which you can easily check. – TigerhawkT3 Oct 10 '15 at 21:56
Er... no, [**`list`**](https://docs.python.org/2/library/functions.html?#list) is a type. – Peter Wood Oct 10 '15 at 21:56
Hmm, I tried the first fix but I think python is having trouble leaving the loop because it's not finishing the script...or perhaps something else is happening. (Edit: I used "lines2" instead of "list") – Dennis Oct 10 '15 at 21:57
@Dennis I'll update answer to cover that, one moment. – Aidan Gomez Oct 10 '15 at 22:00
And why are you doing an augmented list assignment instead of just using `append`? – TigerhawkT3 Oct 10 '15 at 22:00
@AidanGomez Actually, this all worked wonderfully. I had a typo, and now it works great. Thank you! – Dennis Oct 10 '15 at 22:03
@TigerhawkT3 I was mimicking his formatting. I agree that append is more clear and concise. – Aidan Gomez Oct 10 '15 at 22:07
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@Dennis glad it worked! Accept the answer at your convenience. – Aidan Gomez Oct 10 '15 at 22:08
`if len(line) > 0` can also be replaced with `if line`. – TigerhawkT3 Oct 10 '15 at 22:09
@TigerhawkT3 I believe SO has post editing functionality. However, I'm at issue with the modification because the sacrifice of clarity is too great. Especially in an already pseudo-unclear functionality like list comprehension. – Aidan Gomez Oct 10 '15 at 22:11
The use of truthy and falsey values is a basic feature of Python that results in cleaner, more concise code. Avoiding it is the equivalent of using `if myval == False: return False; else: return True` rather than `return myval`. – TigerhawkT3 Oct 10 '15 at 23:42

Difference between unbracketed v. bracketed 'for loop' in python?

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