Understanding IndentationErrors in Python 2.7

Question

I tend to get a LOT of IndentationErrors when writing python code. Sometimes the error will go away when I delete and rewrite the line. Can someone provide a high level explanation of IndentationErrors in python for noob?

Here is an example of a recent indentationError I received while playing CheckIO that will not go away:

def checkpass(data):
     """Checks password for >=10 char + 1 number + 1 LC letter + 1 UC letter"""

     passlist = []
     uclist = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L',
               'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 
               'Y', 'Z']
     lclist = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l',
               'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
               'y', 'z']

     blah blah.. more code

Here is the error that I receive when I call the python script:

$ python passcheck.py
File "passcheck.py", line 5
uclist = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L',
                                                                    ^
IndentationError: unindent does not match any outer indentation level

Answer 1

This kind of issues usually arise when you mix tabs and spaces.

Some editors may show Tab as 4 spaces, but in Python 2.x Tab is interpreted as 8 spaces, so even though the editor shows that multiple lines are indented at the same level, they may not really be indented at the same level for python (Especially if the editor interprets the tab as 4 spaces)

In Python 3.x , you are not allowed to mix tabs and spaces at all, it would throw a syntax error for mixing tabs and spaces in the same python script.

Ideally, you should either use completely tabs or completely spaces throughout your script , 4 spaces is the recommended practice for indentation.