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Diffstat (limited to 'Doc/library/annotationlib.rst')
| -rw-r--r-- | Doc/library/annotationlib.rst | 140 |
1 files changed, 127 insertions, 13 deletions
diff --git a/Doc/library/annotationlib.rst b/Doc/library/annotationlib.rst index ff9578b6088..41c9ce479ff 100644 --- a/Doc/library/annotationlib.rst +++ b/Doc/library/annotationlib.rst @@ -127,16 +127,27 @@ Classes Values are the result of evaluating the annotation expressions. - .. attribute:: FORWARDREF + .. attribute:: VALUE_WITH_FAKE_GLOBALS :value: 2 + Special value used to signal that an annotate function is being + evaluated in a special environment with fake globals. When passed this + value, annotate functions should either return the same value as for + the :attr:`Format.VALUE` format, or raise :exc:`NotImplementedError` + to signal that they do not support execution in this environment. + This format is only used internally and should not be passed to + the functions in this module. + + .. attribute:: FORWARDREF + :value: 3 + Values are real annotation values (as per :attr:`Format.VALUE` format) for defined values, and :class:`ForwardRef` proxies for undefined values. Real objects may contain references to :class:`ForwardRef` proxy objects. .. attribute:: STRING - :value: 3 + :value: 4 Values are the text string of the annotation as it appears in the source code, up to modifications including, but not restricted to, @@ -144,17 +155,6 @@ Classes The exact values of these strings may change in future versions of Python. - .. attribute:: VALUE_WITH_FAKE_GLOBALS - :value: 4 - - Special value used to signal that an annotate function is being - evaluated in a special environment with fake globals. When passed this - value, annotate functions should either return the same value as for - the :attr:`Format.VALUE` format, or raise :exc:`NotImplementedError` - to signal that they do not support execution in this environment. - This format is only used internally and should not be passed to - the functions in this module. - .. versionadded:: 3.14 .. class:: ForwardRef @@ -485,3 +485,117 @@ annotations from the class and puts them in a separate attribute: typ.classvars = classvars # Store the ClassVars in a separate attribute return typ + +Limitations of the ``STRING`` format +------------------------------------ + +The :attr:`~Format.STRING` format is meant to approximate the source code +of the annotation, but the implementation strategy used means that it is not +always possible to recover the exact source code. + +First, the stringifier of course cannot recover any information that is not present in +the compiled code, including comments, whitespace, parenthesization, and operations that +get simplified by the compiler. + +Second, the stringifier can intercept almost all operations that involve names looked +up in some scope, but it cannot intercept operations that operate fully on constants. +As a corollary, this also means it is not safe to request the ``STRING`` format on +untrusted code: Python is powerful enough that it is possible to achieve arbitrary +code execution even with no access to any globals or builtins. For example: + +.. code-block:: pycon + + >>> def f(x: (1).__class__.__base__.__subclasses__()[-1].__init__.__builtins__["print"]("Hello world")): pass + ... + >>> annotationlib.get_annotations(f, format=annotationlib.Format.SOURCE) + Hello world + {'x': 'None'} + +.. note:: + This particular example works as of the time of writing, but it relies on + implementation details and is not guaranteed to work in the future. + +Among the different kinds of expressions that exist in Python, +as represented by the :mod:`ast` module, some expressions are supported, +meaning that the ``STRING`` format can generally recover the original source code; +others are unsupported, meaning that they may result in incorrect output or an error. + +The following are supported (sometimes with caveats): + +* :class:`ast.BinOp` +* :class:`ast.UnaryOp` + + * :class:`ast.Invert` (``~``), :class:`ast.UAdd` (``+``), and :class:`ast.USub` (``-``) are supported + * :class:`ast.Not` (``not``) is not supported + +* :class:`ast.Dict` (except when using ``**`` unpacking) +* :class:`ast.Set` +* :class:`ast.Compare` + + * :class:`ast.Eq` and :class:`ast.NotEq` are supported + * :class:`ast.Lt`, :class:`ast.LtE`, :class:`ast.Gt`, and :class:`ast.GtE` are supported, but the operand may be flipped + * :class:`ast.Is`, :class:`ast.IsNot`, :class:`ast.In`, and :class:`ast.NotIn` are not supported + +* :class:`ast.Call` (except when using ``**`` unpacking) +* :class:`ast.Constant` (though not the exact representation of the constant; for example, escape + sequences in strings are lost; hexadecimal numbers are converted to decimal) +* :class:`ast.Attribute` (assuming the value is not a constant) +* :class:`ast.Subscript` (assuming the value is not a constant) +* :class:`ast.Starred` (``*`` unpacking) +* :class:`ast.Name` +* :class:`ast.List` +* :class:`ast.Tuple` +* :class:`ast.Slice` + +The following are unsupported, but throw an informative error when encountered by the +stringifier: + +* :class:`ast.FormattedValue` (f-strings; error is not detected if conversion specifiers like ``!r`` + are used) +* :class:`ast.JoinedStr` (f-strings) + +The following are unsupported and result in incorrect output: + +* :class:`ast.BoolOp` (``and`` and ``or``) +* :class:`ast.IfExp` +* :class:`ast.Lambda` +* :class:`ast.ListComp` +* :class:`ast.SetComp` +* :class:`ast.DictComp` +* :class:`ast.GeneratorExp` + +The following are disallowed in annotation scopes and therefore not relevant: + +* :class:`ast.NamedExpr` (``:=``) +* :class:`ast.Await` +* :class:`ast.Yield` +* :class:`ast.YieldFrom` + + +Limitations of the ``FORWARDREF`` format +---------------------------------------- + +The :attr:`~Format.FORWARDREF` format aims to produce real values as much +as possible, with anything that cannot be resolved replaced with +:class:`ForwardRef` objects. It is affected by broadly the same Limitations +as the :attr:`~Format.STRING` format: annotations that perform operations on +literals or that use unsupported expression types may raise exceptions when +evaluated using the :attr:`~Format.FORWARDREF` format. + +Below are a few examples of the behavior with unsupported expressions: + +.. code-block:: pycon + + >>> from annotationlib import get_annotations, Format + >>> def zerodiv(x: 1 / 0): ... + >>> get_annotations(zerodiv, format=Format.STRING) + Traceback (most recent call last): + ... + ZeroDivisionError: division by zero + >>> get_annotations(zerodiv, format=Format.FORWARDREF) + Traceback (most recent call last): + ... + ZeroDivisionError: division by zero + >>> def ifexp(x: 1 if y else 0): ... + >>> get_annotations(ifexp, format=Format.STRING) + {'x': '1'} |