PEP: 3107 Title: Function Annotations Version: $Revision$ Last-Modified: $Date$ Author: Collin Winter , Tony Lownds Status: Draft Type: Standards Track Requires: 362 Content-Type: text/x-rst Created: 2-Dec-2006 Python-Version: 3.0 Post-History: Abstract ======== This PEP introduces a syntax for adding arbitrary metadata annotations to Python functions [#functerm]_. Rationale ========= Because Python's 2.x series lacks a standard way of annotating a function's parameters and return values (e.g., with information about a what type a function's return value should be), a variety of tools and libraries have appeared to fill this gap [#tailexamp]_. Some utilise the decorators introduced in "PEP 318", while others parse a function's docstring, looking for annotations there. This PEP aims to provide a single, standard way of specifying this information, reducing the confusion caused by the wide variation in mechanism and syntax that has existed until this point. Fundamentals of Function Annotations ==================================== Before launching into a discussion of the precise ins and outs of Python 3.0's function annotations, let's first talk broadly about what annotations are and are not: 1. Function annotations, both for parameters and return values, are completely optional. 2. Function annotations are nothing more than a way of associating arbitrary Python expressions with various parts of a function at compile-time. By itself, Python does not attach any particular meaning or significance to annotations. Left to its own, Python simply makes these expressions available as the values in the ``__signature__.annotations`` mapping described in `Accessing Function Annotations`_ below. The only way that annotations take on meaning is when they are interpreted by third-party libraries. These annotation consumers can do anything they want with a function's annotations. For example, one library might use string-based annotations to provide improved help messages, like so:: def compile(source: "something compilable", filename: "where the compilable thing comes from", mode: "is this a single statement or a suite?"): ... Another library might be used to provide typechecking for Python functions and methods. This library could use annotations to indicate the function's expected input and return types, possibly something like :: def haul(item: Haulable, *vargs: PackAnimal) -> Distance: ... However, neither the strings in the first example nor the type information in the second example have any meaning on their own; meaning comes from third-party libraries alone. 3. Following from point 2, this PEP makes no attempt to introduce any kind of standard semantics, even for the built-in types. This work will be left to third-party libraries. There is no worry that these libraries will assign semantics at random, or that a variety of libraries will appear, each with varying semantics and interpretations of what, say, a tuple of strings means. The difficulty inherent in writing annotation interpreting libraries will keep their number low and their authorship in the hands of people who, frankly, know what they're doing. Syntax ====== Parameters ---------- Annotations for parameters take the form of optional expressions that follow the parameter name. This example indicates that parameters 'a' and 'c' should both be a ``Number``, while parameter 'b' should both be a ``Mapping``:: def foo(a: Number, b: Mapping, c: Number = 5): ... (Number, Mapping and Sequence are used thoughout this PEP and represent the standard `numeric`_, `mapping`_ and `sequence protocols`_.) In pseudo-grammar, parameters now look like ``identifier [: expression] [= expression]``. That is, type annotations always precede a parameter's default value and both type annotations and default values are optional. Just like how equal signs are used to indicate a default value, colons are used to mark annotations. All annotation expressions are evaluated when the function definition is executed. Annotations for excess parameters (i.e., ``*args`` and ``**kwargs``) are indicated similarly. In the following function definition, ``*args`` is flagged as a list of ``Number``, and ``**kwargs`` is marked as a dict whose keys are strings and whose values are of type ``Sequence``. :: def foo(*args: Number, **kwargs: Sequence): ... Note that, depending on what annotation-interpreting library you're using, the following might also be a valid spelling of the above:: def foo(*args: [Number], **kwargs: {str: Sequence}): ... Only the first, however, has the BDFL's blessing [#blessedexcess]_ as the One Obvious Way. Return Values ------------- The examples thus far have omitted examples of how to annotate the type of a function's return value. This is done like so:: def sum(*args: Number) -> Number: ... The parameter list can now be followed by a literal ``->`` and a Python expression. Like the annotations for parameters, this expression will be evaluated when the function definition is executed. The grammar for function definitions [#grammar]_ is now something like :: funcdef ::= [decorators] "def" funcname "(" [parameter_list] ")" ["->" expression] ":" suite decorators ::= decorator+ decorator ::= "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE dotted_name ::= identifier ("." identifier)* parameter_list ::= (defparameter ",")* ( "*" identifier [":" expression] [, "**" identifier [":" expression] ] | "**" identifier [":" expression] | defparameter [","] ) defparameter ::= parameter [":" expression] ["=" expression] sublist ::= parameter ("," parameter)* [","] parameter ::= identifier | "(" sublist ")" funcname ::= identifier Accessing Function Annotations ============================== Once compiled, a function's annotations are available via the function's ``__signature__`` attribute, introduced by PEP 362. Signature objects include an attribute just for annotations, appropriately called ``annotations``. This attribute is a dictionary, mapping parameter names to an object representing the evaluated annotation expression. There is a special key in the ``annotations`` mapping, ``"return"``. This key is present only if an annotation was supplied for the function's return value. For example, the following annotation:: def foo(a: Number, b: 5 + 6, c: list) -> String: ... would result in a ``__signature__.annotations`` mapping of :: {'a': Number, 'b': 11, 'c': list, 'return': String} The ``return`` key was chosen because it cannot conflict with the name of a parameter; any attempt to use ``return`` as a parameter name would result in a ``SyntaxError``. Implementation ============== A sample implementation has been provided [#implementation]_ by Tony Lownds. Rejected Proposals ================== + The BDFL rejected the author's idea for a special syntax for adding annotations to generators as being "too ugly" [#rejectgensyn]_. + Though discussed early on ([#threadgen]_, [#threadhof]_), including special objects in the stdlib for annotating generator functions and higher-order functions was ultimately rejected as being more appropriate for third-party libraries; including them in the standard library raised too many thorny issues. + Despite considerable discussion about a standard type parameterisation syntax, it was decided that this should also be left to third-party libraries. ([#threadimmlist]_, [#threadmixing]_, [#emphasistpls]_) References and Footnotes ======================== .. [#functerm] Unless specifically stated, "function" is generally used as a synonym for "callable" throughout this document. .. [#tailexamp] The author's typecheck_ library makes use of decorators, while `Maxime Bourget's own typechecker`_ utilises parsed docstrings. .. [#blessedexcess] http://mail.python.org/pipermail/python-3000/2006-May/002173.html .. [#rejectgensyn] http://mail.python.org/pipermail/python-3000/2006-May/002103.html .. _typecheck: http://oakwinter.com/code/typecheck/ .. _Maxime Bourget's own typechecker: http://maxrepo.info/taxonomy/term/3,6/all .. [#threadgen] http://mail.python.org/pipermail/python-3000/2006-May/002091.html .. [#threadhof] http://mail.python.org/pipermail/python-3000/2006-May/001972.html .. [#threadimmlist] http://mail.python.org/pipermail/python-3000/2006-May/002105.html .. [#threadmixing] http://mail.python.org/pipermail/python-3000/2006-May/002209.html .. [#emphasistpls] http://mail.python.org/pipermail/python-3000/2006-June/002438.html .. [#implementation] http://python.org/sf/1607548 .. _numeric: http://docs.python.org/lib/typesnumeric.html .. _mapping: http://docs.python.org/lib/typesmapping.html .. _sequence protocols: http://docs.python.org/lib/typesseq.html .. [#grammar] http://www.python.org/doc/current/ref/function.html Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: