Python 3 support in GRASS

Python versions

• keep compatibility with 2.7 (may still work with 2.6, but we don't care)
• port to work with 3.5

Python components include:

• Python Scripting Library
• PyGRASS
• Temporal Library
• ctypes
• wxGUI

Python Scripting Library

Possible approach:

• functions need to accept unicode and return unicode
• functions wrapping Python Popen class (read_command, run_command, ...) will have parameter encoding
  • encoding=None means expects and returns bytes (the current state)
  • encoding='default' means it takes current encoding using utils._get_encoding()
  • encoding='utf-8' takes whatever encoding user specifies, e.g., utf-8 in this case
  • this is similar to ​Popen class in Python3.6
  • by default encoding='default' to enable expected behavior by users, the following example shows Python3 behavior if we keep using bytes instead of unicode:

# return bytes
ret = read_command('r.what', encoding=None, ...

for item in ret.splitlines():
    line = item.split('|')[3:]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: a bytes-like object is required, not 'str'

# we would have to use:
for item in ret.splitlines():
    line = item.split(b'|')[3:]