58 lines
1.8 KiB
Plaintext
58 lines
1.8 KiB
Plaintext
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PEP: 526
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Title: Variable Declaration Syntax
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Version: $Revision$
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Last-Modified: $Date$
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Author: Ryan Gonzalez <rymg19@gmail.com>, Philip House <phouse512@gmail.com>, Guido van Rossum <guido@python.org>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 09-Aug-2016
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Python-Version: 3.6
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Abstract
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========
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PEP 484 introduced type hints and; In particular, it introduced the notion of
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type comments::
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# a is specified to be a list of ints.
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a = [] # type: List[int]
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# b is a string
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b = None # type: str
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class Cls:
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my_class_attr = True # type: bool
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This PEP aims at adding syntax to Python for declaring the types of variables and
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attributes, instead of expressing them through comments::
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a: List[int] = []
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b: str
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class Cls:
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my_class_attr: ClassAttr[bool] = True
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Rationale
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=========
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Although type comments work well, the fact that they're expressed through
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comments has some downsides:
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- Text editors often highlight comments differently from type declarations.
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- There isn't a way to declare the type of an undefined variable; you need to
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initialize it to ``None`` (e.g. ``a = None # type: int``).
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- Variables declared in a conditional branch are difficult to read::
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if some_value:
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my_var = function() # type: Logger
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else:
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my_var = another_function() # Why isn't there a type here?
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- Since type comments aren't actually part of the language, if a Python script
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wants to parse them, it would require a custom parser instead of just using
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``ast``.
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- It's impossible to retrieve the annotations at runtime outside of attempting to
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find the module's source code and parse it at runtime, which is inelegant, to
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say the least.
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The majority of these issues can be alleviated by making the syntax a core part of
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the language.
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