PEP 572: get rid of side-by-side tables; drop unnecessary TODO (#708)
Alas, the side-by-side tables caused horribly misindented code.
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Guido van Rossum
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pep-0572.rst
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pep-0572.rst
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@ -166,16 +166,8 @@ in order to avoid ambiguities or user confusion:
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foo(x = y := f(x)) # INVALID
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This rule is included to disallow excessively confusing code.
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- TODO: Should we disallow using keyword arguments and top level
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assignment expressions in the same call? E.g.::
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# Should these be invalid?
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foo(x=0, y := f(0))
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bar(x := 0, y = f(x))
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Regardless, ``foo(x := 0)`` should probably be valid (see below).
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This rule is included to disallow excessively confusing code, and
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because parsing keyword arguments is complex enough already.
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- Assignment expressions (even parenthesized or occurring inside other
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constructs) are prohibited in function default values. For example,
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@ -294,7 +286,7 @@ Some examples to clarify what's technically valid or invalid::
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# Valid
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len(lines := f.readlines())
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# Valid (TODO: Should this be disallowed?)
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# Valid
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foo(x := 3, cat='vector')
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# INVALID
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@ -366,121 +358,136 @@ Examples from the Python standard library
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site.py
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^^^^^^^
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+-----------------------------------------------------+------------------------------------------------------------+
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| Current code | Replaced with |
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+-----------------------------------------------------+------------------------------------------------------------+
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| | |
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| :: | :: |
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| | |
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| env_base = os.environ.get("PYTHONUSERBASE", None) | if env_base := os.environ.get("PYTHONUSERBASE", None): |
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| if env_base: | return env_base |
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| return env_base | |
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| | |
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+-----------------------------------------------------+------------------------------------------------------------+
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*env_base* is only used on these lines, putting its assignment on the if
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moves it as the "header" of the block.
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- Current::
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env_base = os.environ.get("PYTHONUSERBASE", None)
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if env_base:
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return env_base
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- Improved::
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if env_base := os.environ.get("PYTHONUSERBASE", None):
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return env_base
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_pydecimal.py
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^^^^^^^^^^^^^
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+----------------------------------------------+-----------------------------------------------------------------------+
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| Current code | Replaced with |
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+----------------------------------------------+-----------------------------------------------------------------------+
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| | |
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| :: | :: |
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| | |
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| if self._is_special: | if self._is_special and (ans := self._check_nans(context=context)): |
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| ans = self._check_nans(context=context) | return ans |
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| if ans: | |
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| return ans | |
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| | |
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+----------------------------------------------+-----------------------------------------------------------------------+
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Avoid nested if and remove one indentation level.
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- Current::
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if self._is_special:
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ans = self._check_nans(context=context)
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if ans:
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return ans
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- Improved::
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if self._is_special and (ans := self._check_nans(context=context)):
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return ans
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copy.py
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^^^^^^^
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Code looks more regular and avoid multiple nested if.
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(See Appendix A for the origin of this example.)
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+---------------------------------------------------------------+---------------------------------------------------------------+
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| Current code | Replaced with |
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+---------------------------------------------------------------+---------------------------------------------------------------+
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| | |
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| :: | :: |
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| | |
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| reductor = dispatch_table.get(cls) | if reductor := dispatch_table.get(cls): |
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| if reductor: | rv = reductor(x) |
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| rv = reductor(x) | elif reductor := getattr(x, "__reduce_ex__", None): |
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| else: | rv = reductor(4) |
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| reductor = getattr(x, "__reduce_ex__", None) | elif reductor := getattr(x, "__reduce__", None): |
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| if reductor: | rv = reductor() |
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| rv = reductor(4) | else: |
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| else: | raise Error("un(deep)copyable object of type %s" % cls) |
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| reductor = getattr(x, "__reduce__", None) | |
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| if reductor: | |
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| rv = reductor() | |
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| else: | |
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| raise Error( | |
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| "un(deep)copyable object of type %s" % cls) | |
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+---------------------------------------------------------------+---------------------------------------------------------------+
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- Current::
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Code looks more regular and avoid multiple nested if.
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reductor = dispatch_table.get(cls)
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if reductor:
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rv = reductor(x)
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else:
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reductor = getattr(x, "__reduce_ex__", None)
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if reductor:
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rv = reductor(4)
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else:
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reductor = getattr(x, "__reduce__", None)
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if reductor:
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rv = reductor()
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else:
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raise Error(
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"un(deep)copyable object of type %s" % cls)
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- Improved::
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if reductor := dispatch_table.get(cls):
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rv = reductor(x)
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elif reductor := getattr(x, "__reduce_ex__", None):
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rv = reductor(4)
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elif reductor := getattr(x, "__reduce__", None):
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rv = reductor()
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else:
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raise Error("un(deep)copyable object of type %s" % cls)
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datetime.py
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^^^^^^^^^^^
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+-----------------------------------------------------+-----------------------------------------------------+
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| Current code | Replaced with |
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+-----------------------------------------------------+-----------------------------------------------------+
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| | |
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| :: | :: |
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| s = _format_time(self._hour, self._minute, | s = _format_time(self._hour, self._minute, |
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| self._second, self._microsecond, | self._second, self._microsecond, |
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| timespec) | timespec) |
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| tz = self._tzstr() | if tz := self._tzstr(): |
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| if tz: | s += tz |
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| s += tz | return s |
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| return s | |
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| | |
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+-----------------------------------------------------+-----------------------------------------------------+
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*tz* is only used for ``s += tz``, moving its assignment inside the if
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helps to show its scope.
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- Current::
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s = _format_time(self._hour, self._minute,
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self._second, self._microsecond,
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timespec)
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tz = self._tzstr()
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if tz:
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s += tz
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return s
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- Improved::
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s = _format_time(self._hour, self._minute,
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self._second, self._microsecond,
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timespec)
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if tz := self._tzstr():
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s += tz
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return s
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sysconfig.py
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^^^^^^^^^^^^
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+------------------------------------+-----------------------------------------+
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| Current code | Replaced with |
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+------------------------------------+-----------------------------------------+
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| :: | :: |
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| while True: | while line := fp.readline(): |
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| line = fp.readline() | if m := define_rx.match(line): |
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| if not line: | n, v = m.group(1, 2) |
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| break | try: |
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| m = define_rx.match(line) | v = int(v) |
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| if m: | except ValueError: |
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| n, v = m.group(1, 2) | pass |
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| try: | vars[n] = v |
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| v = int(v) | elif m := undef_rx.match(line): |
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| except ValueError: | vars[m.group(1)] = 0 |
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| pass | |
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| vars[n] = v | |
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| else: | |
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| m = undef_rx.match(line) | |
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| if m: | |
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| vars[m.group(1)] = 0 | |
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+------------------------------------+-----------------------------------------+
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Calling ``fp.readline()`` in the ``while`` condition and calling
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``.match()`` on the if lines make the code more compact without making
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it harder to understand.
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- Current::
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while True:
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line = fp.readline()
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if not line:
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break
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m = define_rx.match(line)
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if m:
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n, v = m.group(1, 2)
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try:
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v = int(v)
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except ValueError:
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pass
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vars[n] = v
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else:
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m = undef_rx.match(line)
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if m:
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vars[m.group(1)] = 0
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- Improved::
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while line := fp.readline():
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if m := define_rx.match(line):
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n, v = m.group(1, 2)
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try:
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v = int(v)
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except ValueError:
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pass
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vars[n] = v
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elif m := undef_rx.match(line):
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vars[m.group(1)] = 0
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Simplifying list comprehensions
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-------------------------------
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@ -613,6 +620,8 @@ Broadly the same semantics as the current proposal, but spelled differently.
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confusion or require special-casing (eg to forbid assignment within the
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headers of these statements).
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TODO: Explain more why this is bad, since this keeps coming up (it's also readability)
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2. ``EXPR -> NAME``::
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stuff = [[f(x) -> y, x/y] for x in range(5)]
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