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<li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 724</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 724 – Stricter Type Guards</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Rich Chiodo <rchiodo at microsoft.com>, Eric Traut <erictr at microsoft.com>, Erik De Bonte <erikd at microsoft.com></dd> <dt class="field-even">Sponsor:</dt> <dd class="field-even">Jelle Zijlstra <jelle.zijlstra at gmail.com></dd> <dt class="field-odd">Discussions-To:</dt> <dd class="field-odd"><a class="reference external" href="https://discuss.python.org/t/pep-724-stricter-type-guards/34124">Discourse thread</a></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Removed from consideration by sponsor or authors">Withdrawn</abbr></dd> <dt class="field-odd">Type:</dt> <dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-even">Topic:</dt> <dd class="field-even"><a class="reference external" href="../topic/typing/">Typing</a></dd> <dt class="field-odd">Created:</dt> <dd class="field-odd">28-Jul-2023</dd> <dt class="field-even">Python-Version:</dt> <dd class="field-even">3.13</dd> <dt class="field-odd">Post-History:</dt> <dd class="field-odd"><a class="reference external" href="https://mail.python.org/archives/list/typing-sig@python.org/thread/EMUD2D424OI53DCWQ4H5L6SJD2IXBHUL/" title="Typing-SIG thread">30-Dec-2021</a>, <a class="reference external" href="https://discuss.python.org/t/pep-724-stricter-type-guards/34124" title="Discourse thread">19-Sep-2023</a></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#status">Status</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#additional-examples">Additional Examples</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a><ul> <li><a class="reference internal" href="#breaking-change">Breaking change</a></li> </ul> </li> <li><a class="reference internal" href="#how-to-teach-this">How to Teach This</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li> <li><a class="reference internal" href="#rejected-ideas">Rejected Ideas</a><ul> <li><a class="reference internal" href="#stricttypeguard">StrictTypeGuard</a></li> <li><a class="reference internal" href="#typeguard-with-a-second-output-type">TypeGuard with a second output type</a></li> </ul> </li> <li><a class="reference internal" href="#footnotes">Footnotes</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="status"> <h2><a class="toc-backref" href="#status" role="doc-backlink">Status</a></h2> This PEP is withdrawn. The Typing Council was unable to reach consensus on this proposal, and the authors decided to withdraw it. </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <a class="pep reference internal" href="../pep-0647/" title="PEP 647 – User-Defined Type Guards">PEP 647</a> introduced the concept of a user-defined type guard function which returns <code class="docutils literal notranslate">True</code> if the type of the expression passed to its first parameter matches its return <code class="docutils literal notranslate">TypeGuard</code> type. For example, a function that has a return type of <code class="docutils literal notranslate">TypeGuard[str]</code> is assumed to return <code class="docutils literal notranslate">True</code> if and only if the type of the expression passed to its first input parameter is a <code class="docutils literal notranslate">str</code>. This allows type checkers to narrow types when a user-defined type guard function returns <code class="docutils literal notranslate">True</code>. This PEP refines the <code class="docutils literal notranslate">TypeGuard</code> mechanism introduced in <a class="pep reference internal" href="../pep-0647/" title="PEP 647 – User-Defined Type Guards">PEP 647</a>. It allows type checkers to narrow types when a user-defined type guard function returns <code class="docutils literal notranslate">False</code>. It also allows type checkers to apply additional (more precise) type narrowing under certain circumstances when the type guard function returns <code class="docutils literal notranslate">True</code>. </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> User-defined type guard functions enable a type checker to narrow the type of an expression when it is passed as an argument to the type guard function. The <code class="docutils literal notranslate">TypeGuard</code> mechanism introduced in <a class="pep reference internal" href="../pep-0647/" title="PEP 647 – User-Defined Type Guards">PEP 647</a> is flexible, but this flexibility imposes some limitations that developers have found inconvenient for some uses. Limitation 1: Type checkers are not allowed to narrow a type in the case where the type guard function returns <code class="docutils literal notranslate">False</code>. This means the type is not narrowed in the negative (“else”) clause. Limitation 2: Type checkers must use the <code class="docutils literal notranslate">TypeGuard</code> return type if the type guard function returns <code class="docutils literal notranslate">True</code> regardless of whether additional narrowing can be applied based on knowledge of the pre-narrowed type. The following code sample demonstrates both of these limitations. <div class="highlight-python notranslate"><div class="highlight"><pre>def is_iterable(val: object) -> TypeGuard[Iterable[Any]]: return isinstance(val, Iterable) def func(val: int | list[int]): if is_iterable(val): # The type is narrowed to 'Iterable[Any]' as dictated by # the TypeGuard return type reveal_type(val) # Iterable[Any] else: # The type is not narrowed in the "False" case reveal_type(val) # int | list[int] # If "isinstance" is used in place of the user-defined type guard # function, the results differ because type checkers apply additional # logic for "isinstance" if isinstance(val, Iterable): # Type is narrowed to "list[int]" because this is # a narrower (more precise) type than "Iterable[Any]" reveal_type(val) # list[int] else: # Type is narrowed to "int" because the logic eliminates # "list[int]" from the original union reveal_type(val) # int </pre></div> </div> <a class="pep reference internal" href="../pep-0647/" title="PEP 647 – User-Defined Type Guards">PEP 647</a> imposed these limitations so it could support use cases where the return <code class="docutils literal notranslate">TypeGuard</code> type was not a subtype of the input type. Refer to <a class="pep reference internal" href="../pep-0647/" title="PEP 647 – User-Defined Type Guards">PEP 647</a> for examples. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> There are a number of issues where a stricter <code class="docutils literal notranslate">TypeGuard</code> would have been a solution: <ul class="simple"> <li><a class="reference external" href="https://github.com/python/typing/issues/1351">Python typing issue - TypeGuard doesn’t intersect like isinstance</a></li> <li><a class="reference external" href="https://github.com/microsoft/pyright/issues/3450">Pyright issue - TypeGuard not eliminating possibility on branch</a></li> <li><a class="reference external" href="https://github.com/microsoft/pyright/issues/3466">Pyright issue - Type narrowing for Literal doesn’t work</a></li> <li><a class="reference external" href="https://github.com/python/mypy/issues/15305">Mypy issue - TypeGuard is incompatible with exhaustive check</a></li> <li><a class="reference external" href="https://github.com/python/mypy/issues/15520">Mypy issue - Incorrect type narrowing for inspect.isawaitable</a></li> <li><a class="reference external" href="https://github.com/python/typeshed/issues/8009">Typeshed issue - asyncio.iscoroutinefunction is not a TypeGuard</a></li> </ul> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> The use of a user-defined type guard function involves five types: <ul class="simple"> <li>I = <code class="docutils literal notranslate">TypeGuard</code> input type</li> <li>R = <code class="docutils literal notranslate">TypeGuard</code> return type</li> <li>A = Type of argument passed to type guard function (pre-narrowed)</li> <li>NP = Narrowed type (positive)</li> <li>NN = Narrowed type (negative)</li> </ul> <div class="highlight-python notranslate"><div class="highlight"><pre>def guard(x: I) -> TypeGuard[R]: ... def func1(val: A): if guard(val): reveal_type(val) # NP else: reveal_type(val) # NN </pre></div> </div> This PEP proposes some modifications to <a class="pep reference internal" href="../pep-0647/" title="PEP 647 – User-Defined Type Guards">PEP 647</a> to address the limitations discussed above. These limitations are safe to eliminate only when a specific condition is met. In particular, when the output type <code class="docutils literal notranslate">R</code> of a user-defined type guard function is consistent <a class="footnote-reference brackets" href="#isconsistent" id="id1">[1]</a> with the type of its first input parameter (<code class="docutils literal notranslate">I</code>), type checkers should apply stricter type guard semantics. <blockquote> <div><div class="highlight-python notranslate"><div class="highlight"><pre># Stricter type guard semantics are used in this case because # "Kangaroo | Koala" is consistent with "Animal" def is_marsupial(val: Animal) -> TypeGuard[Kangaroo | Koala]: return isinstance(val, Kangaroo | Koala) # Stricter type guard semantics are not used in this case because # "list[T]"" is not consistent with "list[T | None]" def has_no_nones(val: list[T | None]) -> TypeGuard[list[T]]: return None not in val </pre></div> </div> </div></blockquote> When stricter type guard semantics are applied, the application of a user-defined type guard function changes in two ways. <ul class="simple"> <li>Type narrowing is applied in the negative (“else”) case.</li> </ul> <div class="highlight-python notranslate"><div class="highlight"><pre>def is_str(val: str | int) -> TypeGuard[str]: return isinstance(val, str) def func(val: str | int): if not is_str(val): reveal_type(val) # int </pre></div> </div> <ul class="simple"> <li>Additional type narrowing is applied in the positive “if” case if applicable.</li> </ul> <div class="highlight-python notranslate"><div class="highlight"><pre>def is_cardinal_direction(val: str) -> TypeGuard[Literal["N", "S", "E", "W"]]: return val in ("N", "S", "E", "W") def func(direction: Literal["NW", "E"]): if is_cardinal_direction(direction): reveal_type(direction) # "Literal[E]" else: reveal_type(direction) # "Literal[NW]" </pre></div> </div> The type-theoretic rules for type narrowing are specified in the following table. <table class="docutils align-default"> <thead> <tr class="row-odd"><th class="head"></th> <th class="head">Non-strict type guard</th> <th class="head">Strict type guard</th> </tr> </thead> <tbody> <tr class="row-even"><td>Applies when</td> <td>R not consistent with I</td> <td>R consistent with I</td> </tr> <tr class="row-odd"><td>NP is ..</td> <td>R</td> <td>A∧R</td> </tr> <tr class="row-even"><td>NN is ..</td> <td>A</td> <td>A∧¬R</td> </tr> </tbody> </table> In practice, the theoretic types for strict type guards cannot be expressed precisely in the Python type system. Type checkers should fall back on practical approximations of these types. As a rule of thumb, a type checker should use the same type narrowing logic – and get results that are consistent with – its handling of “isinstance”. This guidance allows for changes and improvements if the type system is extended in the future. </section> <section id="additional-examples"> <h2><a class="toc-backref" href="#additional-examples" role="doc-backlink">Additional Examples</a></h2> <code class="docutils literal notranslate">Any</code> is consistent <a class="footnote-reference brackets" href="#isconsistent" id="id2">[1]</a> with any other type, which means stricter semantics can be applied. <div class="highlight-python notranslate"><div class="highlight"><pre> # Stricter type guard semantics are used in this case because # "str" is consistent with "Any" def is_str(x: Any) -> TypeGuard[str]: return isinstance(x, str) def test(x: float | str): if is_str(x): reveal_type(x) # str else: reveal_type(x) # float </pre></div> </div> </section> <section id="backwards-compatibility"> <h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2> This PEP proposes to change the existing behavior of <code class="docutils literal notranslate">TypeGuard</code>. This has no effect at runtime, but it does change the types evaluated by a type checker. <div class="highlight-python notranslate"><div class="highlight"><pre>def is_int(val: int | str) -> TypeGuard[int]: return isinstance(val, int) def func(val: int | str): if is_int(val): reveal_type(val) # "int" else: reveal_type(val) # Previously "int | str", now "str" </pre></div> </div> This behavioral change results in different types evaluated by a type checker. It could therefore produce new (or mask existing) type errors. Type checkers often improve narrowing logic or fix existing bugs in such logic, so users of static typing will be used to this type of behavioral change. We also hypothesize that it is unlikely that existing typed Python code relies on the current behavior of <code class="docutils literal notranslate">TypeGuard</code>. To validate our hypothesis, we implemented the proposed change in pyright and ran this modified version on roughly 25 typed code bases using <a class="reference external" href="https://github.com/hauntsaninja/mypy_primer">mypy primer</a> to see if there were any differences in the output. As predicted, the behavioral change had minimal impact. The only noteworthy change was that some <code class="docutils literal notranslate"># type: ignore</code> comments were no longer necessary, indicating that these code bases were already working around the existing limitations of <code class="docutils literal notranslate">TypeGuard</code>. <section id="breaking-change"> <h3><a class="toc-backref" href="#breaking-change" role="doc-backlink">Breaking change</a></h3> It is possible for a user-defined type guard function to rely on the old behavior. Such type guard functions could break with the new behavior. <div class="highlight-python notranslate"><div class="highlight"><pre>def is_positive_int(val: int | str) -> TypeGuard[int]: return isinstance(val, int) and val > 0 def func(val: int | str): if is_positive_int(val): reveal_type(val) # "int" else: # With the older behavior, the type of "val" is evaluated as # "int | str"; with the new behavior, the type is narrowed to # "str", which is perhaps not what was intended. reveal_type(val) </pre></div> </div> We think it is unlikely that such user-defined type guards exist in real-world code. The mypy primer results didn’t uncover any such cases. </section> </section> <section id="how-to-teach-this"> <h2><a class="toc-backref" href="#how-to-teach-this" role="doc-backlink">How to Teach This</a></h2> Users unfamiliar with <code class="docutils literal notranslate">TypeGuard</code> are likely to expect the behavior outlined in this PEP, therefore making <code class="docutils literal notranslate">TypeGuard</code> easier to teach and explain. </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2> A reference <a class="reference external" href="https://github.com/microsoft/pyright/commit/9a5af798d726bd0612cebee7223676c39cf0b9b0">implementation</a> of this idea exists in pyright. To enable the modified behavior, the configuration flag <code class="docutils literal notranslate">enableExperimentalFeatures</code> must be set to true. This can be done on a per-file basis by adding a comment: <div class="highlight-python notranslate"><div class="highlight"><pre># pyright: enableExperimentalFeatures=true </pre></div> </div> </section> <section id="rejected-ideas"> <h2><a class="toc-backref" href="#rejected-ideas" role="doc-backlink">Rejected Ideas</a></h2> <section id="stricttypeguard"> <h3><a class="toc-backref" href="#stricttypeguard" role="doc-backlink">StrictTypeGuard</a></h3> A new <code class="docutils literal notranslate">StrictTypeGuard</code> construct was proposed. This alternative form would be similar to a <code class="docutils literal notranslate">TypeGuard</code> except it would apply stricter type guard semantics. It would also enforce that the return type was consistent <a class="footnote-reference brackets" href="#isconsistent" id="id5">[1]</a> with the input type. See this thread for details: <a class="reference external" href="https://github.com/python/typing/discussions/1013#discussioncomment-1966238">StrictTypeGuard proposal</a> This idea was rejected because it is unnecessary in most cases and added unnecessary complexity. It would require the introduction of a new special form, and developers would need to be educated about the subtle difference between the two forms. </section> <section id="typeguard-with-a-second-output-type"> <h3><a class="toc-backref" href="#typeguard-with-a-second-output-type" role="doc-backlink">TypeGuard with a second output type</a></h3> Another idea was proposed where <code class="docutils literal notranslate">TypeGuard</code> could support a second optional type argument that indicates the type that should be used for narrowing in the negative (“else”) case. <div class="highlight-python notranslate"><div class="highlight"><pre>def is_int(val: int | str) -> TypeGuard[int, str]: return isinstance(val, int) </pre></div> </div> This idea was proposed <a class="reference external" href="https://github.com/python/typing/issues/996">here</a>. It was rejected because it was considered too complicated and addressed only one of the two main limitations of <code class="docutils literal notranslate">TypeGuard</code>. Refer to this <a class="reference external" href="https://mail.python.org/archives/list/typing-sig@python.org/thread/EMUD2D424OI53DCWQ4H5L6SJD2IXBHUL">thread</a> for the full discussion. </section> </section> <section id="footnotes"> <h2><a class="toc-backref" href="#footnotes" role="doc-backlink">Footnotes</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="isconsistent" role="doc-footnote"> <dt class="label" id="isconsistent">[1] (<a href='#id1'>1</a>, <a href='#id2'>2</a>, <a href='#id5'>3</a>) </dt> <dd><a class="pep reference internal" href="../pep-0483/#summary-of-gradual-typing" title="PEP 483 – The Theory of Type Hints § Summary of gradual typing">PEP 483’s discussion of is-consistent</a></aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0724.rst">https://github.com/python/peps/blob/main/peps/pep-0724.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0724.rst">2025-02-01 08:55:40 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#status">Status</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#additional-examples">Additional Examples</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a><ul> <li><a class="reference internal" href="#breaking-change">Breaking change</a></li> </ul> </li> <li><a class="reference internal" href="#how-to-teach-this">How to Teach This</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li> <li><a class="reference internal" href="#rejected-ideas">Rejected Ideas</a><ul> <li><a class="reference internal" href="#stricttypeguard">StrictTypeGuard</a></li> <li><a class="reference internal" href="#typeguard-with-a-second-output-type">TypeGuard with a second output type</a></li> </ul> </li> <li><a class="reference internal" href="#footnotes">Footnotes</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0724.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>