file.write() not permanently writing to file python

Question

I have a text file of names, all of which have three spaces at the end of them, which I would like to remove. When I print these names in python, I get output like the follows:

Adeline Panella Â Winifred Aceto Â  See Weckerly Â Daniell Hildebrand Â Betsey Coulter Â  #there are about 1000 of these names

To remove the extra spaces, I wrote the following script:

import os script_directory = os.path.dirname(__file__) file = open(os.path.join(script_directory, "assets/data/names.txt"), 'r') potential_names = file.read().splitlines() potential_names = list(filter(None, potential_names)) for item in potential_names: print(item) item = item[:-3] print(item) file.close() file = open(os.path.join(script_directory, "assets/data/names.txt"), 'w') for item in potential_names: file.write("{}\n".format(item)) file.close()

It appears to function as expected, as the output is as follows:

Adeline Panella Â  Adeline Panella Winifred Aceto Â  Winifred Aceto See Weckerly Â  See Weckerly Daniell Hildebrand Â  Daniell Hildebrand Betsey Coulter Â  Betsey Coulter

HOWEVER: When I run the script a second time, the output is exactly the same, and when I examine the text file, the three spaces at the end remain there. How can I permanently remove this extra spacing?

Here’s something that might be neater: gist.github.com/anonymous/6828cdfe74564472160e — Ry-
– Ry- ♦, Commented Dec 7, 2015 at 1:58

paxdiablo · Accepted Answer · 2015-12-07 05:21:51Z

for item in potential_names: print(item) item = item[:-3] print(item)

When you change item on that third line above, it does not reflect back to the potential_names collection, it simply changes item. That's why it appears to be modifying the string⁽¹⁾.

However, later, when you process the collection:

for item in potential_names:

that's the original contents of the collection you're outputting.

One way to get around this is to simply construct a new list with the final three characters removed from each item:

potential_names = [x[:-3] for x in potential_names]

⁽¹⁾ Python is generally considered a pure object-oriented language because everything is an object to which names refer.

That has certain limitations in that the expression item = '12345'; item = item[:-3] doesn't change the value of the underlying '12345' string, it creates a new string and changes the value of the item reference to refer to it.

That aspect of the language was a real eye-opener once I figured out how it works.

Ah, that makes sense. I am relatively new to python and assumed item would be a reference to the item inside the list, not a local copy of the item.
@WookieCoder: It is a reference to the item inside the list. item = item[:-3] assigns a new reference to item.

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