I spend a lot of time flipping back and forth between Python 2.x and 3.x: I use different versions for different projects, talk to people about different versions, explore differences between the two, and paste the output of REPL sessions into chat windows. I also like to keep long-running REPL sessions. These two activities in combination became quite confusing, and I’d often forget which version I was using.

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>>> print some_var
  File "<stdin>", line 1
    print some_var
                 ^
SyntaxError: invalid syntax
>>> # *swears*

After the hundredth time I made this mistake, I decided to modify my prompt to make it always obvious which version was which, even in long-running REPL sessions. You can do this by creating a file to be run when Python starts up. Add this line to your .bashrc:

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export PYTHONSTARTUP=~/mystartupscript.py

Then in mystartupscript.py:

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import sys
if sys.version_info.major == 3:
    sys.ps1 = "PY3 >>> "
    sys.ps2 = "PY3 ... "

This makes it obvious when you’re about to slip up:

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PY3 >>> for value in giant_collection:
PY3 ...     print(value)
PY3 ...

I’ve also add this line to mystartupscript.py to bite the bullet and start using print as a function everywhere:

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from __future__ import print_function

This has no effect in Python3.x, but will move 2.x to the new syntax.