38 lines
1.4 KiB
ReStructuredText
38 lines
1.4 KiB
ReStructuredText
.. _func_annotations:
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Function annotations
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====================
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Function annotations are a piece of syntax introduced in Python 3.0 that was
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not backported to Python 2.x. (See PEP 3107:
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http://www.python.org/dev/peps/pep-3107/). They cause Python 2 to raise a
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SyntaxError.
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To rewrite Python 3 code with function annotations to be compatible with both
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Python 3 and Python 2, you can replace the annotation syntax with a dictionary
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called ``__annotations__`` as an attribute on your functions. For example, code
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such as this::
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def _parse(self, filename: str, dir='.') -> list:
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pass
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can be re-expressed like this::
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def _parse(self, filename, dir='.'):
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pass
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_parse.__annotations__ = {'filename': str, 'return': list}
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As described in PEP 3107, the annotation for a function's return value
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corresponds to the ``'return'`` key in the dictionary.
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(Note that PEP 3107 describes annotations as belonging to a
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``func_annotations`` attribute. This attribute was renamed in Python 3.2 to
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``__annotations__``.)
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Be aware that some libraries that consume function annotations, such as
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`Reticulated <https://github.com/mvitousek/reticulated>`_, have their own
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semantics for supporting earlier Python versions, such as decorators. If you
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are using such a library, please use its own mechanism for providing
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compatibility with earlier Python versions, rather than the generic equivalent
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above.
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