Standard library

In the Python standard library, formatting a type name or the type name of an object is a common operation to format an error message and to implement a __repr__() method. There are different ways to format a type name which give different outputs.

Example with the datetime.timedelta type:

• The type short name (type.__name__) and the type qualified name (type.__qualname__) are 'timedelta'.
• The type module (type.__module__) is 'datetime'.
• The type fully qualified name is 'datetime.timedelta'.
• The type representation (repr(type)) contains the fully qualified name: <class 'datetime.timedelta'>.

PyErr_Format(PyExc_TypeError, "globals must be a dict, not %.100s",
             Py_TYPE(globals)->tp_name);

PyErr_Format(PyExc_TypeError,
             "calling %R should have returned an instance "
             "of BaseException, not %R",
             type, Py_TYPE(value));

Using PyTypeObject.tp_name is inconsistent with Python

The PyTypeObject.tp_name member is different depending on the type implementation:

• Static types and heap types in C: tp_name is the type fully qualified name.
• Python class: tp_name is the type short name (type.__name__).

So using Py_TYPE(obj)->tp_name to format an object type name gives a different output depending if a type is implemented in C or in Python.

It goes against PEP 399 “Pure Python/C Accelerator Module Compatibility Requirements” principles which recommends code behaves the same way if written in Python or in C.

Example:

$ python3.12
>>> import _datetime; c_obj = _datetime.date(1970, 1, 1)
>>> import _pydatetime; py_obj = _pydatetime.date(1970, 1, 1)
>>> my_list = list(range(3))

>>> my_list[c_obj]  # C type
TypeError: list indices must be integers or slices, not datetime.date

>>> my_list[py_obj]  # Python type
TypeError: list indices must be integers or slices, not date

The error message contains the type fully qualified name (datetime.date) if the type is implemented in C, or the type short name (date) if the type is implemented in Python.

Limited C API

The Py_TYPE(obj)->tp_name code cannot be used with the limited C API, since the PyTypeObject members are excluded from the limited C API.

The type name should be read using PyType_GetName(), PyType_GetQualName() and PyType_GetModule() functions which are less convenient to use.

Truncating type names in C

In 1998, when the PyErr_Format() function was added, the implementation used a fixed buffer of 500 bytes. The function had the following comment:

/* Caller is responsible for limiting the format */

In 2001, the function was modified to allocate a dynamic buffer on the heap. Too late, the practice of truncating type names, like using the %.100s format, already became a habit, and developers forgot why type names are truncated. In Python, type names are not truncated.

Truncating type names in C but not in Python goes against PEP 399 “Pure Python/C Accelerator Module Compatibility Requirements” principles which recommends code behaves the same way if written in Python or in C.

See the issue: Replace %.100s by %s in PyErr_Format(): the arbitrary limit of 500 bytes is outdated (2011).

Add PyType_GetFullyQualifiedName() function

Add the PyType_GetFullyQualifiedName() function to get the fully qualified name of a type: similar to f"{type.__module__}.{type.__qualname__}", or type.__qualname__ if type.__module__ is not a string or is equal to "builtins" or is equal to "__main__".

API:

PyObject* PyType_GetFullyQualifiedName(PyTypeObject *type)

On success, return a new reference to the string. On error, raise an exception and return NULL.

Add PyType_GetModuleName() function

Add the PyType_GetModuleName() function to get the module name of a type (type.__module__ string). API:

PyObject* PyType_GetModuleName(PyTypeObject *type)

On success, return a new reference to the string. On error, raise an exception and return NULL.

Add formats to PyUnicode_FromFormat()

Add the following formats to PyUnicode_FromFormat():

• %N formats the fully qualified name of a type, similar to PyType_GetFullyQualifiedName(type); N stands for type Name.
• %T formats the type fully qualified name of an object’s type, similar to PyType_GetFullyQualifiedName(Py_TYPE(obj)); T stands for object Type.
• %#N and %#T: the alternative form uses the colon separator (:), instead of the dot separator (.), between the module name and the qualified name.

For example, the existing code using tp_name:

PyErr_Format(PyExc_TypeError,
             "__format__ must return a str, not %.200s",
             Py_TYPE(result)->tp_name);

can be replaced with the %T format:

PyErr_Format(PyExc_TypeError,
             "__format__ must return a str, not %T", result);

Advantages of the updated code:

• Safer C code: avoid Py_TYPE() which returns a borrowed reference.
• The PyTypeObject.tp_name member is no longer read explicitly: the code becomes compatible with the limited C API.
• The formatted type name no longer depends on the type implementation.
• The type name is no longer truncated.

Note: The %T format is used by time.strftime(), but not by printf().

Formats Summary

Recommend using the type fully qualified name

The type fully qualified name is recommended in error messages and in __repr__() methods in new C code.

In non-trivial applications, it is likely to have two types with the same short name defined in two different modules, especially with generic names. Using the fully qualified name helps identifying the type in an unambiguous way.

Recommend not truncating type names

Type names should not be truncated in new C code. For example, the %.100s format should be avoided: use the %s format instead (or %T format in C).

Add type.__fully_qualified_name__ attribute

Add type.__fully_qualified_name__ read-only attribute, the fully qualified name of a type: similar to f"{type.__module__}.{type.__qualname__}", or type.__qualname__ if type.__module__ is not a string or is equal to "builtins" or is equal to "__main__".

The type.__repr__() is left unchanged, it only omits the module if the module is equal to "builtins".

This change was rejected by the Steering Council:

We can see the usefulness of the C API changes proposed by the PEP and would likely accept those changes as is.

We see less justification for the Python level changes. We especially question the need for __fully_qualified_name__.

Thomas Wouters added:

If there really is a desire for formatting types the exact same way the C API does it, a utility function would make more sense to me, personally, than type.__format__, but I think the SC could be persuaded given some concrete use-cases.

Add type.__format__() method

Add type.__format__() method with the following formats:

• N formats the type fully qualified name (type.__fully_qualified_name__); N stands for Name.
• #N (alternative form) formats the type fully qualified name using the colon (:) separator, instead of the dot separator (.), between the module name and the qualified name.

Examples using f-string:

>>> import datetime
>>> f"{datetime.timedelta:N}"  # fully qualified name
'datetime.timedelta'
>>> f"{datetime.timedelta:#N}" # fully qualified name, colon separator
'datetime:timedelta'

The colon (:) separator used by the #N format eliminates guesswork when you want to import the name, see pkgutil.resolve_name(), python -m inspect command line interface, and setuptools entry points.

This change was rejected by the Steering Council.

Change str(type)

The type.__str__() method can be modified to format a type name differently. For example, it can return the type fully qualified name.

The problem is that it’s a backward incompatible change. For example, enum, functools, optparse, pdb and xmlrpc.server modules of the standard library have to be updated. test_dataclasses, test_descrtut and test_cmd_line_script tests have to be updated as well.

See the pull request: type(str) returns the fully qualified name.

Add !t formatter to get an object type

Use f"{obj!t:T}" to format type(obj).__fully_qualified_name__, similar to f"{type(obj):T}".

When the !t formatter was proposed in 2018, Eric Smith was strongly opposed to this; Eric is the author of the f-string PEP 498 “Literal String Interpolation”.

Add formats to str % args

It was proposed to add formats to format a type name in str % arg. For example, add the %T format to format a type fully qualified name.

Nowadays, f-strings are preferred for new code.

Other ways to format type names in C

The printf() function supports multiple size modifiers: hh (char), h (short), l (long), ll (long long), z (size_t), t (ptrdiff_t) and j (intmax_t). The PyUnicode_FromFormat() function supports most of them.

Proposed formats using h and hh length modifiers:

• %hhT formats type.__name__.
• %hT formats type.__qualname__.
• %T formats type.__fully_qualified_name__.

Length modifiers are used to specify the C type of the argument, not to change how an argument is formatted. The alternate form (#) changes how an argument is formatted. Here the argument C type is always PyObject*.

Other proposed formats:

• %Q
• %t.
• %lT formats type.__fully_qualified_name__.
• %Tn formats type.__name__.
• %Tq formats type.__qualname__.
• %Tf formats type.__fully_qualified_name__.

Having more options to format type names can lead to inconsistencies between different modules and make the API more error prone.

About the %t format, printf() now uses t as a length modifier for ptrdiff_t argument.

The following APIs to be used to format a type:

Use %T format with Py_TYPE(): pass a type

It was proposed to pass a type to the %T format, like:

PyErr_Format(PyExc_TypeError, "object type name: %T", Py_TYPE(obj));

The Py_TYPE() functions returns a borrowed reference. Just to format an error, using a borrowed reference to a type looks safe. In practice, it can lead to crash. Example:

import gc
import my_cext

class ClassA:
    pass

def create_object():
     class ClassB:
          def __repr__(self):
                self.__class__ = ClassA
                gc.collect()
                return "ClassB repr"
     return ClassB()

obj = create_object()
my_cext.func(obj)

where my_cext.func() is a C function which calls:

PyErr_Format(PyExc_ValueError,
             "Unexpected value %R of type %T",
             obj, Py_TYPE(obj));

PyErr_Format() is called with a borrowed reference to ClassB. When repr(obj) is called by the %R format, the last reference to ClassB is removed and the class is deallocated. When the %T format is proceed, Py_TYPE(obj) is already a dangling pointer and Python does crash.

Other proposed APIs to get a type fully qualified name

• Add type.__fullyqualname__ attribute: name without underscore between words. Several dunders, including some of the most recently added ones, include an underscore in the word: __class_getitem__, __release_buffer__, __type_params__, __init_subclass__ and __text_signature__.
• Add type.__fqn__ attribute: FQN name stands for Fully Qualified Name.
• Add type.fully_qualified_name() method. Methods added to type are inherited by all types and so can affect existing code.
• Add a function to the inspect module. Need to import the inspect module to use it.

Include the __main__ module in the type fully qualified name

Format type.__fully_qualified_name__ as f"{type.__module__}.{type.__qualname__}", or type.__qualname__ if type.__module__ is not a string or is equal to "builtins". Do not treat the __main__ module differently: include it in the name.

Existing code such as type.__repr__(), collections.abc and unittest modules format a type name with f'{obj.__module__}.{obj.__qualname__}' and only omit the module part if the module is equal to builtins.

Only the traceback and pdb modules also omit the module if it’s equal to "builtins" or "__main__".

The type.__fully_qualified_name__ attribute omits the __main__ module to produce shorter names for a common case: types defined in a script run with python script.py. For debugging, the repr() function can be used on a type, it includes the __main__ module in the type name. Or use f"{type.__module__}.{type.__qualname__}" format to always include the module name, even for the "builtins" module.

Example of script:

class MyType:
    pass

print(f"name: {MyType.__fully_qualified_name__}")
print(f"repr: {repr(MyType)}")

Output:

name: MyType
repr: <class '__main__.MyType'>