Python 2/3 Compatibility

This guideline documents coding conventions to achieve hybrid Python-2.7/Python-3.5 compatibility in Sahana.

It's a working document that will be added to as we move towards full Python-3 compatibility.

Syntax

No print statements

Don't use the print statement anywhere:

print "example" # deprecated

You shouldn't use the print-function either, because it can clash with uWSGI:

print("example") # not good

...but it can be tolerated in CLI scripts which don't run in the WSGI environment.

Best option for debug output is to use sys.stderr.write:

import sys
sys.stderr.write("example\n") # better

...or the logger (as it can be configured globally to write to a log file instead of the system console):

current.log.debug("example") # even better

Use as-syntax for catching exceptions

When catching exceptions in a variable, don't use the comma-syntax:

try:
    ...
except Exception, e: # deprecated
    ...

Instead, use the as-keyword:

try:
    ...
except Exception as e: # new standard
    ...

Usage

No implicit package-relative imports

Python-3 does not search for modules relative to the current module in the same package - unless explicitly indicated by leading . or .. in the module path.

from s3datetime import s3_format_datetime # inside modules/s3, not working in Python-3

Python-2.7 would search relative to the current module, but on the other hand, it supports the explicit-relative syntax as well.

So we decide that only explicit paths shall be used in imports.

To import a module in the same package (e.g. within s3), either use explicit-relative syntax:

from .s3datetime import s3_format_datetime # inside modules/s3, preferred variant

...or an absolute path relative to modules (or the global python path):

from s3.s3datetime import s3_format_datetime # inside modules/s3, acceptable alternative

Outside of modules/s3, you should always import from the top-level of the s3 package (because the package structure may change over time):

from s3 import s3_format_datetime # outside modules/s3

Alternative Imports

As the locations and names of some libraries have changed in Python-3, we use the compatibility module (modules/s3compat.py) to implement suitable alternatives. Similarily, the compat module provides alternatives for other objects such as types, functions and certain common patterns.

Where an object name is provided by modules/s3compat.py, it must be imported from there if used.

The following objects are provided by s3compat:

Name	Import	Comments/Caveats
PY2	from s3compat import PY2	Constant indicating whether we're currently running on Py2, should only be used if alternatives cannot be generalized
StringIO	from s3compat import StringIO
basestring	from s3compat import basestring