Whitespace problems with python

Question

I'm currently using Komodo Edit in Windows 7, however, I have experienced this problem on my Mac with TextWrangler. For whatever reason I'm getting some kind of whitespace error which is a huge problem when I'm writing in Python. For example, everything appears to be properly tabbed, but Komodo is currently giving me an "Ambiguous whitespace" error

//How it appears in my editor
def SaveList(self, directory):
    templist = []
    templistbox2 = []
    for n,i in enumerate(self.listbox2.get(0,END)): 
       templistbox2.insert(n, re.sub(r'^[0-9]*[.]',"",str(i)))
    for filename in sorted(os.listdir(directory)):
        self.templist.insert(i, filename)
        print filename #whitespace error here

Considering I've experienced this with two different editors on both windows and Mac, I'm wondering if there's some setting I don't know about, or if I'm doing something wrong.

Could you have tabs in the indentation? Maybe the setting you want is about converting tabs to space. — f p, Jan 30 '13 at 13:07

unutbu · Accepted Answer · 2013-01-30T13:32:27.187

When I copy your code to a file, test.py, and run

cat -A test.py

I see

//How it appears in my editor$
def SaveList(self, directory):$
    templist = []$
    templistbox2 = []$
    for n,i in enumerate(self.listbox2.get(0,END)): $
       templistbox2.insert(n, re.sub(r'^[0-9]*[.]',"",str(i)))$
    for filename in sorted(os.listdir(directory)):$
        self.templist.insert(i, filename)$
^I    print filename #whitespace error here$

which indicates there is a tab (represented by ^I) followed by four spaces on the last line.

I'm not sure what the equivalent tool on Windows would be, but the Mac should have the cat -A command. It will show you where the tabs versus spaces are.

There is a program called reindent.py which will convert tabs to spaces for you:

reindent.py test.py

On Unix there is also a unexpand command which converts spaces to tabs.

Most Python programmers use spaces rather than tabs for indentation. Most of the Python code you find on the web will use spaces rather than tabs.

Your editor may be adding tabs, but if you took a snippet of code from the web, your file may now contain both tabs and spaces.

It is easiest to go with the flow and adopt the spaces-as-indentation convention, so you will not have to reformat other people's code so much.

By the way, adopting the spaces-as-indentation convention does not mean having to press SPACE 4 times for each indentation level. Your editor should have a configuration option which makes pressing TAB insert 4 spaces.

+1 It is obvious that there had to be tabs mixed with spaces, but showing the OP how to find them is a nice bonus! — Veger, Jan 30 '13 at 13:12
Thanks for the help, I'll give it a try. Edit - this is very strange. I removed the tab where I was getting the error, and pressed space 4 times and it went away. So, tab caused an error but 4 spaces didn't. I'm confused, shouldn't they be the exact same spacing? — user1104854, Jan 30 '13 at 13:35
@user1104854: They *look* the same on an editor which formats one TAB the same way as 4 spaces, but not all editors are the same. Python does not assume any relationship between tabs and spaces, so it has no clue what indentation level 1 tab and 4 spaces is supposed to mean. In Python the rule is **never** mix tabs and spaces for indentation. — unutbu, Jan 30 '13 at 13:50

score 2 · Answer 2 · edited Jan 30 '13 at 14:06

This is a common problem in Python. The following advice may help you:

1) Never mix spaces and tabs. For new projects, use spaces rather than tabs See PEP8 My recommendation is to use 4 spaces.

2) Change the default values for tabs length in Komodo, in order to detect the mixes more easily . Press the Edit > Preferences menu, then in the Editor settings:

Uncheck, Prefer Tab characters over spaces.
Use 4 for Number of spaces for indent
use a different value (8 for example) for width of each tab character

3) The reindent.py script in C:\Python2x\Tools\Scripts\ can help you to reindent files properly

-d (--dryrun)   Dry run.   Analyze, but don't make any changes to, files.
-r (--recurse)  Recurse.   Search for all .py files in subdirectories too.
-n (--nobackup) No backup. Does not make a ".bak" file before reindenting.
-v (--verbose)  Verbose.   Print informative msgs; else no output.
-h (--help)     Help.      Print this usage information and exit.

Change Python (.py) files to use 4-space indents and no hard tab characters.
Also trim excess spaces and tabs from ends of lines, and remove empty lines
at the end of files.  Also ensure the last line ends with a newline.

I hope it helps

score 1 · Answer 3 · answered Apr 13 '13 at 08:30

Since no one else mentioned it, there is an easy way to make whitespace visible right inside Komodo itself. You don't need to use any external tools.

Simply select View/View Whitespace from the menu, or use the keyboard shortcut Ctrl+Shift+8 on Windows. Repeat to turn it back off.

While you're on the View menu, check out some of the other useful features there. I find the Indentation Guides especially helpful and leave them turned on all the time. View EOL Markers is also useful from time to time.

Whitespace problems with python

3 Answers3