Python 3 compatibility/conversions

Official compatibility: Odoo 11 will be the first LTS release to introduce Python 3 compatibility, starting with Python 3.5. It will also be the first LTS release to drop official support for Python 2.

Rationale: Python 3 has been around since 2008, and all Python libraries used by the official Odoo distribution have been ported and are considered stable. Most supported platforms have a Python 3.5 package, or a similar way to deploy it. Preserving dual compatibility is therefore considered unnecessary, and would represent a significant overhead in testing for the lifetime of Odoo 11.

Python 2 and Python 3 are somewhat different language, but following backports, forward ports and cross-compatibility library it is possible to use a subset of Python 2 and Python 3 in order to have a system compatible with both.

Here are a few useful steps or reminders to make Python 2 code compatible with Python 3.

References/useful documents:

Versions Support

A cross compatible Odoo would only support Python 2.7 and Python 3.5 and above: Python 2.7 backported some Python 3 features, and Python 2 features were reintroduced in various Python 3 in order to make conversion easier. Python 3.6 adds great features (f-strings, …) and performance improvements (ordered compact dicts) but does not seem to reintroduce compatibility features whereas:

  • Python 3.5 reintroduced % for bytes/bytestrings (PEP 461)
  • Python 3.4 has no specific compatibility improvement but is the lowest P3 version for PyLint
  • Python 3.3 reintroduced the «u» prefix for proper (unicode) strings
  • Python 3.2 made range views more list-like (backported to 2.7)and reintroduced callable

Semantics changes

Dict & set iteration order («Hash Randomisation»)

In Python 2, the iteration order depends on the value’s hash (modulo the collection’s capacity and conflict resolution), which provides a spec-undefined but implementation-defined order. While that’s not supposed to happen, it turns out code may depend on the specific order of iteration over a hash collection (dict or set).

Python 3.3 enables hash randomisation by default (this can be optionally enabled on previous versions including Python 2 by providing the -R command-line parameter), which means the order of iteration changes from one run to the next.

When discovered, this can be fixed by one of:

  • making iteration steps properly independent (removing the dependency of order of iteration)
  • using different checking method (e.g. when serialising sets or dictionaries and checking against the specific serialised value)
  • fixing dependencies
  • using a collections.OrderedDict or odoo.tools.misc.OrderedSet instead of a regular one, they guarantee order of iteration is order of insertion
  • sorting the collection’s items before iterating over them (this may require adding some sort of iteration key to the items)

Moved and removed

Standard Library Modules

Python 3 reorganised, moved or removed a number of modules in the standard library:

  • StringIO and cStringIO were removed, you can use io.BytesIO and io.StringIO to replace them in a cross-version manner (io.BytesIO for binary data, io.StringIO for text/unicode data).

  • urllib, urllib2 and urlparse were redistributed across urllib.parse and urllib.request.

    Since requests and werkzeug are already hard dependencies of Odoo, replace urllib[2].urlopen/urllib2.Request uses by requests, and urlparse and a few utilty functions (urllib.quote, urllib.urlencode) are available through werkzeug.urls, a backport of Python 3’s urllib.parse.

  • cgi.escape (HTML escaping) is deprecated in Python 3, prefer Odoo’s own odoo.tools.misc.html_encode().
  • Most of types’s content has been stripped out in Python 3: only «internal» interpreter types (e.g. CodeType, FrameType, …) have been left in, other types can be obtained directly from the corresponding builtin or by getting the type() of a literal value.

Absolute Imports (PEP 328)

In Python 2 import statements are ambiguous: if a file a.py contains import b, the import system will first check if there’s a b.py file next to it before checking if there is a package called that on the PYTHONPATH.

Furthermore if a sibling file is named the same as top-level package, the library becomes inaccessible to both the file itself ans siblings, this has actually happened in Odoo with odoo.tools.mimetypes.

Additionally, relative imports allow navigating «up» the tree by using multiple leading ..

Exception Handlers

In Python 2, except statements are of the form:

except Exception[, name]:

or:

except (Exception1, Exception2)[, name]:

But because the name is optional, this gets confusing and people can stumble into the first form when trying for the second and write:

except Exception1, Exception:

which will not yield the expected result.

Python 3 changes this syntax to:

except Exception[ as name]:

or:

except (Exception1, Exception2)[ as name]:

This form was implemented in Python 2.5 and is thus compatible across the board.

Operators & keywords

These two operators were long recommended against/deprecated in Python 2, Python 3 removed them from the language.

In Python 2, exec is a statement/keyword. Much like print, it’s been converted to a builtin function in Python 3. However because the Python 2 version can take a tuple parameter it is easy to convert the odd exec statement to the following cross-language forms:

exec(source)
exec(source, globals)
exec(source, globals, locals)

List/iteration builtins and methods

In Python 3, a number of builtins and methods formerly returning lists were converted to return iterators or views, with the corresponding redundant methods or functions having been removed entirely:

  • In Python 3, map, filter and zip return iterators, itertools.imap, itertools.ifilter and itertools.izip have been removed.

  • In Python 3, dict.keys, dict.values and dict.items return views rather than lists, and the iter* and view* methods have been removed.

builtins

cmp

The cmp builtin function has been removed from Python 3.

  • Most of its uses are in cmp= parameters to sort functions where it can usually be replaced by a key function.
  • Other uses found were obtaining the sign of an item (cmp(item, 0)), this can be replicated using the standard library’s math.copysign e.g. math.copysign(1, item) will return 1.0 if item is positive and -1.0 if item is negative.

execfile

execfile(path) has been removed completely from Python 3 but it is trivially replaceable in all cases by:

exec(open(path, 'rb').open())

of a variant thereof (see exec changes for details)

file

The file builtin has been removed in Python 3. Generally, it can just be replaced by the open builtin, although you may want to use io.open which is more flexible and better handles the binary/text dichotomy, a big issue in cross-version Python.

long

In Python 2, integers can be either int or long. Python 3 unifies this under the single int type.

  • the L suffix on numbers is unsupported in Python 3, and unnecessary in Python 2 as «overflowing» integer literals will implicitly instantiate long.
  • in Python 2, a call to int() will implicitly create a long object if necessary.
  • type-testing is the last and bigger issue as in Python 2 long is not a subtype of int (nor the reverse), and isinstance(value, (int, long)) is thus generally necessary to catch all integrals.

reduce

In Python 3, reduce has been demoted from builtin to functools.reduce. However this is because most uses of ``reduce`` can be replaced by ``sum``, ``all``, ``any`` or a list comprehension for a more readable and faster result.

It is easy enough to just add from functools import reduce to the file and compatible with Python 2.6 and later, but consider whether you get better code by replacing it with some other method altogether.

xrange

In Python 3, range() behaves the same as Python 2’s xrange.

For cross-version code, you can just use range() everywhere: while this will incur a slight allocation cost on Python 2, Python 3’s range supports the entire Sequence protocol and thus behaves very much like a regular list or tuple.

Removed/renamed methods

in for dicts was introduced in Python 2.3, leading to has_key being redundant, and removed in Python 3.

Minor syntax changes

  • the ability to unpack a parameter (in the parameter declaration list) has been removed in Python 3 e.g.:

    def foo((bar, baz), qux):
    …

    is now invalid

  • octal literals must be prefixed by 0o (or 0O). Following the C family, in Python 2 an octal literal simply has a leading 0, which can be confusing and easy to get wrong when e.g. padding for readability (e.g. 0013 would be the decimal 11 rather than 13).

    In Python 3, leading zeroes followed by neither a 0 nor a period is an error, octal literals now follow the hexadecimal convention with a 0o prefix.

Bytes/String/Text: The Big One

The most impactful Python 3 change by far is to the text model: for historical reasons the distinction Python 2’s bytestrings (bytes/str) and text strings (unicode) is fuzzy and it will try to implicitly convert between one and the other using the ASCII encoding.

Python 3 changes this, it removes the implicit conversions, removes APIs which contribute to the fuzz and tends to strictly segregate other to work on either bytes or text.

This is fundamentally good and mostly sensible, but it means lots of breakage:

the builtins

Python 3 removes both unicode and basestring, and str now corresponds to text strings (the old unicode) with bytes being bytestrings in both languages 1.

Both versions have the following prefixes for string literals:

  • b'foo' is a bytestring (bytes object).
  • 'foo' is that version’s str type, which may be either a bytestring or a text string 2.
  • u'foo' is that version’s text string.

For best cross-version compatibility you should avoid unprefixed string literals unless you specifically need a «native string» 2.

open

On both P2 and P3, open defaults to returning native strings in default («text») mode, however in P3 that means it actually decodes the file’s bytes using whatever encoding was set up (default: UTF-8) while on Python 2 it has no concept of encoding.

Using open in binary mode provides bytestrings on both versions and works fine. To read text files, use io.open and provide an explicit encoding.

base64

base64 is a bytes->bytes conversion. bytes->bytes codecs were removed from the «native» encoding/decoding system which is now exclusively for bytes<->text conversions: text is encoded to bytes and bytes are decoded to text.

csv

csv is a fairly vicious one: not only is it not a very good format, the Python 2 and Python 3 versions of the library are text-model incompatible in significant ways:

  • Python 2’s CSV only works on ascii-compatible byte streams (it has no encoding support at all) and extracts bytestring values
  • Python 3’s CSV only works on text streams and extract text values
  • And io doesn’t provide «native string» streaming facilities.

However with respect to Odoo it turns out most or all uses of csv fit inside a model of byte stream to and from text values.

The latter is thus a model implemented by cross-version wrappers odoo.tools.pycompat.csv_reader() and odoo.tools.pycompat.csv_writer(): they take a UTF-8 byte stream and read or write text values.

[1] with the caveat that Python 3 makes them less text-y and more byte-y e.g. in Python 2 b"foo"[0] is b"f", but in Python 3 it’s 102 (the value of the first byte), you’ll want to slice bytestrings for compatibility.
[2] (1, 2) this is important because some API/contexts take a native string rather than either bytes or text. The csv module of the standard library is one such problematic API (it is also notoriously problematic for its terrible support of non-ascii-compatible encodings in Python 2). email.message_from_string is an other one.