Basics on Empty List in Python

Question

I am a newbie in Python, and I only have been programming in the language for the third day. I convert from Matlab because I am graduating from university and I just cannot afford the heavy cost of the software.

I am trying to add several filenames to a list, which fails. The following is my code:

    import os
    dirname = r"D:\MyFiles"
    temp = os.listdir(dirname)
    fl = []
    for fn in temp:
        if fn.startswith('L0116'):
            fl = fl.append(fn)

What I don't understand is, the variable fl when initiated with [] returns as the list type, but then I fail to append the list with the above loop. I recheck my code using interactive prompt and I found out that the type changes to str and fails to append more.

I am stuck at this and I tried to google for about an hour without any clear clue to how I should do this.

Is there a better way? Or which part did I do it wrong?

Thank you so much for your help!

Celada · Accepted Answer · 2012-01-13 09:09:25Z

2

The append() method mutates the list in place. You should just append fn like this:

fl.append(fn)

don't try to assign the result of fl.append(fn) (which is None, by the way)

Also, don't use r"foo" strings for filenames. They are designed for use with regular expressions and using them for filenames or other kinds of strings will bite you. For example, you cannot end such a string with a backslash.

answered Jan 13, 2012 at 9:09

Celada

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8 Comments

Harry MacDowel Over a year ago

Thanks! Concept-problem after all.

Harry MacDowel Over a year ago

However, if I do not add the r in front, the listdir function will read the the character after '\' and render errors. Is there any way around this?

Celada Over a year ago

Use "D:\\MyFiles". Yes, it's ugly to have to double every pathname separator. Considering the backslash has been used as a string escape character for many decades, it was a very poor choice to use it also as a pathname separator. But that's what you're stuck with now if you're stuck with MS Windows.

Harry MacDowel Over a year ago

Thank you again! And yes!! ARGH. The more I code the more I hate MS Windows, though I admit that Win7 looks good.

Lennart Regebro Over a year ago

@Celada: r'' is perfectly acceptable for filenames. It is not "designed for regexps". It is designed as a string that doesn't have escapes.

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Ocaso Protal · Accepted Answer · 2012-01-13 09:07:26Z

2

append works inplace, so no need to assign it to the list itself (fl = fl.append(fn)) just do fl.append(fn)

answered Jan 13, 2012 at 9:07

Ocaso Protal

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Comments

machnine · Accepted Answer · 2016-01-03 23:30:57Z

2

List comprehension:

import os
dirname = r"D:\MyFiles"
fl = [fn for fn in os.listdir(dirname) if fn.startswith('L0116')]

answered Jan 3, 2016 at 23:30

machnine

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Comments

James Ravenscroft · Accepted Answer · 2012-01-13 09:07:39Z

0

List append in python does not return a type, instead, you simply call the method like so:

import os
dirname = r"D:\MyFiles"
temp = os.listdir(dirname)
fl = []
for fn in temp:
    if fn.startswith('L0116'):
        fl.append(fn)

I haven't tested this code, but it should work. Your logic was resetting the list every time.

answered Jan 13, 2012 at 9:07

James Ravenscroft

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