Love maybe lines: does not raise an error; it just doesn’t work…

Hello there,

I could make it work with this code:

love_maybe_lines_stripped = [love_maybe_lines[i].strip() for i in range(len(love_maybe_lines))]

However, I do not understand why this code does not work -it does not raise an error; it just doesn’t work…

love_maybe_lines_stripped = [love_maybe_lines[i].strip() for i in love_maybe_lines]

Since each i is a string, it should work, right?



x = [y.strip() for y in love_maybe_lines]

Note that y is the entire line. As it is the actual value, we do not use an index.

Thank you so much for replying.

I understand now that this code:

for i in love_maybe_line:
i =i.strip()

does not work because I’m trying to change a string, which is immutable. Is that it?

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It doesn’t work because in has no binding to the elements. i cannot be changed because this type of loop is Read Only and i being the variable associated with for is protected.

It is correct that a string cannot be changed, but we can create an expression from the string and save it as a new assignment to the same variable.

s = s + "s"

That’s because we simply gave s a new reference, the value of the expression on the right side of the assignment.

There are moving pieces in a comprehension, just as in list.append() for loop logic. Some find it easier to learn A before B, and others, B before A. For these iterators, it should be clear what .append() does… It mutates an object. Comprehensions create objects. All the logic to create it is contained in the delimiters that Python recognizes, whether list, tuple, set, dict, etc.

[expression_to_append for element in iterable]

The value we append can be the element modified by any valid expression.

We cannot access the object we are creating which means we cannot check for duplicates. If duplicates are an issue, build a set comprehension which can then be cast to a tuple or list or dict.

In your program task you have an array where each element is a string. It’s because it is a str object that we have a strip() method. It is an attribute of the the str class, and inherited by all objects of the type.

The process is meant to reduce all the strings to be free of white space either leading or trailing. That is why we can do this,

stripped_lines = [line.strip() for line in lines]

Now to see that in a loop,

stripped_lines = []
for line in lines:

Note that we are not attempting to change line, only use it in the construction of a new value, which is the object we create in the expression.

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You are the best, Roy

Many thanks,


MTF, thank you very much for this explanation. I was struggling to understand why my strip method was not appending to the list, but you made it clear we are using .strip() in the construction of a new value.

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