Convert Python 2 into Python 3 (with Python 2 style execution)

Question

I have a code in Python 2 which I would like to convert to Python 3, however the behaviour of the code MUST remain exactly the same as on Python 2 interpreter.

Especially division must behave like on Python 2 interpreter.

I tried with futurize stage 1 and 2 but division remains just / on the code, which generates errors on execution because returned number is of type float instead of type int (which was a default behaviour for code executed on Python 2 interpreter).

EDIT: 3 // 2 = 1.0 which is float in Python 3. Is there a way to return int?

AKX · Accepted Answer · 2019-04-09 16:02:58Z

1

Futurize/Modernize/2to3 can't statically analyze which divisions are integer division.

You will need to manually go through division operations in the converted code and figure out which should use // for integer division and which should use / for real division.

Naturally, your code base's test cases should find any regressions here. ;)

answered Apr 9, 2019 at 16:02

AKX

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6 Comments

Ziemo Over a year ago

But if all Python 2 divisions with / are type int - there should be a futurize flag to change all those / into //. At least on your own risk.

AKX Over a year ago

Then you should be able to just search-and-replace for ` / ` -> ` // ` (taking care not to change anything that might be in string literals or similar of course).

Ziemo Over a year ago

that's why I'm asking - to not convert strings with simple replace. I have 100+/ matches in 11 files and I'm sure I want to change all / div to //.

AKX Over a year ago

Well, if it helps, PyCharm's search-and-replace feature has a filter for "Except Comments and String Literals" :)

Ziemo Over a year ago

well, that's may be helpful, but I can't believe there is no way to do this (on your own risk) by Futurize/Modernize/2to3.

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Grant Williams · Accepted Answer · 2019-04-09 16:04:57Z

0

The integer division operator is // in python 3.

>>> 3 / 2
1.5
>>> 3 // 2
1

edit: The only way I know of to get compatible behavior from both versions of python is to futurize the python 2 / operation and to convert any instances that expect integer division in the python 2 program to instead use // which works for integer divisions in both python 2 and 3.

edited Apr 9, 2019 at 16:04

answered Apr 9, 2019 at 15:59

Grant Williams

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4 Comments

Ziemo Over a year ago

thanks ;) But how can I forced futurize to change all those divisions? Because if I simply replace / with // it can generate other errors

Grant Williams Over a year ago

if you want code compatible with both python 2 and 3 i'd use the python 3 operation for / instead. You can futurize the division in the python 2 program and update the instances where you need integer division to use // which also does integer division in python 2

Ziemo Over a year ago

3 // 2 = 1.0 which is float. Is there a way to return int?

martineau Over a year ago

@Ziemo: 3 // 2 => 1 in Python 3 (result type int)

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