Porting tips¶

This section highlights some of the known incompatible changes made in Python that could break Python scripts and modules that used to work in prior versions. The sections are split into retroactive changes made to all Python releases, and information specific to every Python branch (compared to the previous one).

This guide is by no means considered complete. If you can think of other problems you’ve hit while porting your packages, please let me know and I will update it.

Retroactive changes¶

bpo43882: urlsplit now strips LF, CR and HT characters¶

Changed in: 2.7.18_p9, 3.6.13_p3, 3.7.10_p3, 3.8.9_p2, 3.9.4_p1

Historically, various urllib.parse methods have passed special characters such as LF, CR and HT through into the split URL components. This could have resulted in various exploits if Python programs did not validate the resulting components and used them verbatim.

bpo43882 attempted to address the issue by making urllib.parse strip the three aforementioned characters from the output of its functions. This fixed one class of potential issues but at the same time opened another can of worms. For example, URL validators that used to check for dangerous special characters in the split URL components stopped working correctly. In the best case, the URL were now sanitized instead of being rejected. In the worst, the original unparsed URL with dangerous characters started being passed through. See e.g. django PR#14349 for an example of impact and a fix.

Behavior before:

>>> urllib.parse.urlparse('https://example.com/bad\nurl')
ParseResult(scheme='https', netloc='example.com', path='/bad\nurl', params='', query='', fragment='')

Behavior after:

>>> urllib.parse.urlparse('https://example.com/bad\nurl')
ParseResult(scheme='https', netloc='example.com', path='/badurl', params='', query='', fragment='')

Python 3.11¶

Generator-based coroutine removal (asyncio.coroutine)¶

Support for generator-based coroutines has been deprecated since Python 3.8, and is finally removed in 3.11. This usually results in the following error:

AttributeError: module 'asyncio' has no attribute 'coroutine'

The recommended solution is to use PEP 492 coroutines. They are available since Python 3.5. This means replacing the @asyncio.coroutine decorator with async def keyword, and yield from with await.

For example, the following snippet:

@asyncio.coroutine
def foo():
    yield from asyncio.sleep(5)

would become:

async def foo():
    await asyncio.sleep(5)

inspect.getargspec() and inspect.formatargspec() removal¶

The inspect.getargspec() (deprecated since Python 3.0) and inspect.formatargspec() (deprecated since Python 3.5) functions are both removed in Python 3.11.

The inspect.getargspec() function provides a legacy interface to inspect the signature of callables. It is replaced by the object-oriented inspect.signature() API (available since Python 3.3), or a mostly compatible inspect.getfullargspec() function (available since Python 3.0).

For example, a trivial function would yield the following results:

>>> def foo(p1, p2, /, kp3, kp4 = 10, kp5 = None, *args, **kwargs):
...     pass
...
>>> inspect.getargspec(foo)
ArgSpec(args=['p1', 'p2', 'kp3', 'kp4', 'kp5'],
        varargs='args',
        keywords='kwargs',
        defaults=(10, None))
>>> inspect.getfullargspec(foo)
FullArgSpec(args=['p1', 'p2', 'kp3', 'kp4', 'kp5'],
            varargs='args',
            varkw='kwargs',
            defaults=(10, None),
            kwonlyargs=[],
            kwonlydefaults=None,
            annotations={})
>>> inspect.signature(foo)
<Signature (p1, p2, /, kp3, kp4=10, kp5=None, *args, **kwargs)>

The named tuple returned by inspect.getfullargspec() starts with the same information, except that the key used to hold the name of ** parameter is varkw rather than keywords. inspect.signature() returns a Signature object.

Both of the newer functions support keyword-only arguments and type annotations:

>>> def foo(p1: int, p2: str, /, kp3: str, kp4: int = 10,
...         kp5: float = None, *args, k6: str, k7: int = 12,
...         k8: float, **kwargs) -> float:
...     pass
...
>>> inspect.getfullargspec(foo)
FullArgSpec(args=['p1', 'p2', 'kp3', 'kp4', 'kp5'],
            varargs='args',
            varkw='kwargs',
            defaults=(10, None),
            kwonlyargs=['k6', 'k7', 'k8'],
            kwonlydefaults={'k7': 12},
            annotations={'return': <class 'float'>,
                         'p1': <class 'int'>,
                         'p2': <class 'str'>,
                         'kp3': <class 'str'>,
                         'kp4': <class 'int'>,
                         'kp5': <class 'float'>,
                         'k6': <class 'str'>,
                         'k7': <class 'int'>,
                         'k8': <class 'float'>})
>>> inspect.signature(foo)
<Signature (p1: int, p2: str, /, kp3: str, kp4: int = 10,
            kp5: float = None, *args, k6: str, k7: int = 12,
            k8: float, **kwargs) -> float>

One notable difference between inspect.signature() and the two other functions is that the latter always include the ‘self’ argument of method prototypes, while the former skips it if the method is bound to an object. That is:

>>> class foo:
...     def x(self, bar):
...         pass
...
>>> inspect.getargspec(foo.x)
ArgSpec(args=['self', 'bar'], varargs=None, keywords=None, defaults=None)
>>> inspect.getargspec(foo().x)
ArgSpec(args=['self', 'bar'], varargs=None, keywords=None, defaults=None)
>>> inspect.signature(foo.x)
<Signature (self, bar)>
>>> inspect.signature(foo().x)
<Signature (bar)>

The inspect.formatargspec() function provides a pretty-formatted argument spec from the tuple returned by inspect.getfullargspec() (or inspect.getargspec()). It is replaced by stringification of Signature objects:

>>> def foo(p1: int, p2: str, /, kp3: str, kp4: int = 10,
...         kp5: float = None, *args, k6: str, k7: int = 12,
...         k8: float, **kwargs) -> float:
...     pass
...
>>> inspect.formatargspec(*inspect.getfullargspec(foo))
'(p1: int, p2: str, kp3: str, kp4: int=10, kp5: float=None, '
'*args, k6: str, k7: int=12, k8: float, **kwargs) -> float'
>>> str(inspect.signature(foo))
'(p1: int, p2: str, /, kp3: str, kp4: int = 10, kp5: float = None, '
'*args, k6: str, k7: int = 12, k8: float, **kwargs) -> float'

Python 3.10¶

configure: No package ‘python-3.1’ found¶

automake prior to 1.16.3 wrongly recognized Python 3.10 as 3.1. As a result, build with Python 3.10 fails:

checking for python version... 3.1
checking for python platform... linux
checking for python script directory... ${prefix}/lib/python3.10/site-packages
checking for python extension module directory... ${exec_prefix}/lib/python3.10/site-packages
checking for PYTHON... no
configure: error: Package requirements (python-3.1) were not met:

No package 'python-3.1' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables PYTHON_CFLAGS
and PYTHON_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
Error: Process completed with exit code 1.

To resolve this in ebuild, you need to autoreconf with the Gentoo distribution of automake:

inherit autotools

# ...

src_prepare() {
    default
    eautoreconf
}

The upstream fix is to create new distfiles using automake-1.16.3+.

distutils.sysconfig deprecation¶

Upstream intends to remove distutils by Python 3.12. Python 3.10 starts throwing deprecation warnings for various distutils modules. The distutils.sysconfig is usually easy to port.

The following table summarizes replacements for common path getters.

distutils.sysconfig call

sysconfig replacement

get_python_inc(False)

get_path("include")

get_python_inc(True)

get_path("platinclude")

get_python_lib(False, False)

get_path("purelib")

get_python_lib(True, False)

get_path("platlib")

get_python_lib(False, True)

get_path("stdlib")

get_python_lib(True, True)

get_path("platstdlib")

For both functions, omitted parameters default to False. There is no trivial replacement for the variants with prefix argument.

Python 3.9¶

base64.encodestring / base64.decodestring removal¶

Python 3.9 removes the deprecated base64.encodestring() and base64.decodestring() functions. While they were deprecated since Python 3.1, many packages still use them today.

The drop-in Python 3.1+ replacements are base64.encodebytes() and base64.decodebytes(). Note that contrary to the names, the old functions were simply aliases to the byte variants in Python 3 and required the arguments to be bytes anyway.

If compatibility with Python 2 is still desired, then the byte variants ought to be called on 3.1+ and string variants before that. The old variants accept both byte and unicode strings on Python 2.

Example compatibility import:

import sys

if sys.version_info >= (3, 1):
    from base64 import encodebytes as b64_encodebytes
else:
    from base64 import encodestring as b64_encodebytes

Note that the base64 module also provides b64encode() and b64decode() functions that were not renamed. b64decode() can be used as a drop-in replacement for decodebytes(). However, b64encode() does not insert newlines to split the output like encodebytes() does, and instead returns a single line of base64-encoded data for any length of output.

Python 3.8¶

python-config and pkg-config no longer list Python library by default¶

Until Python 3.7, the python-X.Y pkg-config file and python-config tool listed the Python library. Starting with 3.8, this is no longer the case. If you are building Python extensions, this is fine (they are not supposed to link directly to libpython).

If you are building programs that need to embed the Python interpreter, new python-X.Y-embed pkg-config file and --embed parameter are provided for the purpose.

$ pkg-config --libs python-3.7
-lpython3.7m
$ pkg-config --libs python-3.8

$ pkg-config --libs python-3.8-embed
-lpython3.8

To achieve backwards compatibility, you should query python-X.Y-embed first and fall back to python-X.Y.

Replacing the toml package¶

The old toml package is no longer maintained. It was last released in November 2020 and it was never updated to implement TOML 1.0. The recommended alternatives are:

the built-in tomllib module (since Python 3.11) with fallback to tomli package for reading TOML files
the tomli-w package for writing TOML files
the tomlkit package for editing already existing TOML files while preserving style

Porting to tomllib/tomli without toml fallback¶

Using a combination of tomllib and tomli is the recommended approach for packages that only read TOML files, or both read and write them but do not need to preserve style. The tomllib module is available since Python 3.11, while tomli versions providing a compatible API are compatible with Python 3.6 and newer.

The key differences between toml and tomllib/tomli are:

the load() function accepts only a file object open for reading in binary mode whereas toml expects a path or a file object open for reading in text mode
the exception raised for invalid input is named TOMLDecodeError where it is named TomlDecodeError in toml

For example, the following code:

import toml

try:
    d1 = toml.load("in1.toml")
except toml.TomlDecodeError:
    d1 = None

with open("in2.toml", "r") as f:
    d2 = toml.load(f)

d3 = toml.loads('test = "foo"\n')

would normally be written as:

import sys

if sys.version_info >= (3, 11):
    import tomllib
else:
    import tomli as tomllib

try:
    # tomllib does not accept paths
    with open("in1.toml", "rb") as f:
        d1 = tomllib.load(f)
# the exception uses uppercase "TOML"
except tomllib.TOMLDecodeError:
    d1 = None

# the file must be open in binary mode
with open("in2.toml", "rb") as f:
    d2 = tomllib.load(f)

d3 = tomllib.loads('test = "foo"\n')

The following dependency string:

dependencies = [
    "toml",
]

would be replaced by:

dependencies = [
    "tomli >= 1.2.3; python_version < '3.11'",
]

Porting to tomllib/tomli with toml fallback¶

If upstream insists on preserving compatibility with EOL versions of Python, it is possible to use a combination of tomllib, tomli and toml. Unfortunately, the incompatibilites in API need to be taken into consideration.

For example, a backwards compatible code for loading a TOML file could look like the following:

import sys

try:
    if sys.version_info >= (3, 11):
        import tomllib
    else:
        import tomli as tomllib

    try:
        with open("in1.toml", "rb") as f:
            d1 = tomllib.load(f)
    except tomllib.TOMLDecodeError:
        d1 = None
except ImportError:
    import toml

    try:
        with open("in1.toml", "r") as f:
            d1 = toml.load(f)
    except toml.TomlDecodeError:
        d1 = None

In this case, the dependency string becomes more complex:

dependencies = [
    "tomli >= 1.2.3; python_version >= '3.6' and python_version < '3.11'",
    "toml; python_version < '3.6'",
]

Porting to tomli-w¶

tomli-w provides a minimal module for dumping TOML files.

The key differences between toml and tomli-w are:

the dump() function takes a file object open for writing in binary mode whereas toml expected a file object open for writing in text mode
providing a custom encoder instance is not supported