New submission from Andres Ayala <killer...@gmail.com>:
I have to read and write many files for an old fortran program where the
elements are coded as integer (so 0 is the X, 1 is the Y...) and I have some
tags associated to each one.
So I tried to create an Enum with some additional fields:
class Coord(Enum):
def __init__(self, value, label, unit):
super().__init__()
self._value_ = value
self.label = label
self.unit = unit
PX = (0, 'P.X', 'km')
PY = (1, 'P.Y', 'km')
VX = (2, 'V.X', 'km/s')
VY = (3, 'V.Y', 'km/s')
This almost work:
>>> for element in CoordOk:
... print("{0}: {0.label}[{0.unit}] = {0.value}".format(element))
CoordOk.PX: P.X[km] = 0
CoordOk.PY: P.Y[km] = 1
CoordOk.VX: V.X[km/s] = 2
CoordOk.VY: V.Y[km/s] = 3
But fails recovering the coordinate from the value:
>>> print(Coord(0))
Traceback (most recent call last):
File "/home/killerrex/tmp/demo.py", line 33, in <module>
print(Coord(0))
File "/usr/lib64/python3.6/enum.py", line 291, in __call__
return cls.__new__(cls, value)
File "/usr/lib64/python3.6/enum.py", line 533, in __new__
return cls._missing_(value)
File "/usr/lib64/python3.6/enum.py", line 546, in _missing_
raise ValueError("%r is not a valid %s" % (value, cls.__name__))
ValueError: 0 is not a valid Coord
Because the internal value of the enum is (0, 'P.X', 'km') and not 0
I found that it is possible to do what I am trying to do with:
class CoordOk(Enum):
def __new__(cls, value, label, unit):
obj = object.__new__(cls)
obj._value_ = value
obj.label = label
obj.unit = unit
return obj
PX = (0, 'P.X', 'km')
PY = (1, 'P.Y', 'km')
VX = (2, 'V.X', 'km/s')
VY = (3, 'V.Y', 'km/s')
Following the latest doc from Enum. Although it is not so intuitive (defining
__new__ is not as easy as __init__)
To make the first proposal work fine is enough with moving the line 221 of
enum.py after the line 224 (see the patch)
I haven't found problems with the change but obviously my little reduced tests
are not representative at all.
And even if it works there may be good reasons to not allow changing the
_value_ in the init.
So mi enhancement proposal is either:
Add an example like the CoordOk to the documentation to help others to find how
to change the _value_ (and an explanation on why is better not to do it in the
__init__ would be perfect)
or
Change the line enum.py so it is possible to update the _value_ in the __init__
method and if you want use the Coord example in the docs.
Thanks!
----------
components: Library (Lib)
files: enum_allow_change_value_in_init.patch
keywords: patch
messages: 316256
nosy: killerrex
priority: normal
severity: normal
status: open
title: Defining __init__ in enums
type: enhancement
versions: Python 3.6
Added file:
https://bugs.python.org/file47575/enum_allow_change_value_in_init.patch
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue33437>
_______________________________________
_______________________________________________
Python-bugs-list mailing list
Unsubscribe:
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
