Python gotchas # 1 : Indentation Tabs and Spaces

Python gotchas - Indentation

or how not to get caught out when you start using python

At first glance python seems very familiar, especially if you have used other procedural languages such as C or Java - but actually Python is different - in some cases very different, and those differences can trip you up as you progress along your python journey. In this series of occasional posts, I am going to cover some of those gotchas.

Indentation, Tabs and spaces 

As you no doubt already know by now, Python uses the end of a line (in most cases) to delimit statements and uses indentation to identify blocks of code in a loop or function, and Python allows you to use either Tabs or spaces to create your indentations and even mix both Tabs and spaces on the same line but there is a hidden gotcha.

Tabs and spaces look alike in most editors - but if you indent one line with Tabs - and the next with spaces - Python will complain with an IndentationError or a TabError even though the lines will seem to be perfectly aligned in your editor.

This can be such a problem that the official style guide (PEP 008) recommends only using spaces for new code, and setting your editor to insert 4 spaces when the tab key is pressed, and you should only use Tabs to stay compliant with existing code which you might be changing. Make sure you get your editor set up before you start coding - as finding and replacing those tab characters might be tricky when you have several 100 lines of code.

The use of indentation provides another thing to trip over if you are not wary. Since Indentation is important for structure the code you can change the behaviour of your code by indenting or un-indenting a line :

Example 1

flag = False
if flag :
    print "Flag is True"
print "Continuing"

Example 2

flag = False
if flag :
    print "Flag is True"
    print "Continuing"

Example 1 and 2 above are identical, apart from one having the last line indented, but Example 1 generates a line of output, and Example 2 doesn't. You can easily see how this could drastically change how your program behaves, and there isn't a error generated, as both of these are completely valid.

Special care should be taken if you copy and paste code into help sites such as stackoverflow which will often remove formatting, make your code invalid, and make it difficult for other people to help you