[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:

There was an effort to make it so `_generate_next_value_` could be defined last 
and still work correctly -- unfortunately, it could not handle the more common 
case of using `auto()` with the default `_generate_next_value_`:

  class I(Enum):
  first = auto()
  second = first + 2# this line would fail

Closing the ticket.  Thank you everyone!

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stage: patch review -> resolved
status: open -> closed

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[miss-islington]

Change by miss-islington :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[miss-islington]

Change by miss-islington :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Change by Ethan Furman :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Change by Ethan Furman :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:

Not yet.  I want to investigate the idea Ankesh Saha had some more.

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[శ్రీనివాస్ రెడ్డి తాటిపర్తి]

Srinivas  Reddy Thatiparthy(శ్రీనివాస్ రెడ్డి తాటిపర్తి) 
 added the comment:

We can close this?

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:


New changeset ebd44003c9e206755e5e28716242ed8941495a62 by Miss Islington (bot) 
in branch '3.7':
bpo-40025: Require _generate_next_value_ to be defined before members (GH-19762)
https://github.com/python/cpython/commit/ebd44003c9e206755e5e28716242ed8941495a62


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:


New changeset b5ecbf02e4dbdea6d1c9a6d7189137f76e70c073 by Miss Islington (bot) 
in branch '3.8':
bpo-40025: Require _generate_next_value_ to be defined before members(GH-19763)
https://github.com/python/cpython/commit/b5ecbf02e4dbdea6d1c9a6d7189137f76e70c073


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[hongweipeng]

Change by hongweipeng :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[miss-islington]

Change by miss-islington :


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pull_request: https://github.com/python/cpython/pull/19763

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[miss-islington]

Change by miss-islington :


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nosy_count: 8.0 -> 9.0
pull_requests: +19083
pull_request: https://github.com/python/cpython/pull/19762

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:


New changeset d9a43e20facdf4ad10186f820601c6580e1baa80 by Ethan Onstott in 
branch 'master':
bpo-40025: Require _generate_next_value_ to be defined before members (GH-19098)
https://github.com/python/cpython/commit/d9a43e20facdf4ad10186f820601c6580e1baa80


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Onstott]

Ethan Onstott  added the comment:

Ankesh, that is the expected behavior as no patch has been merged yet.

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ankesh Saha]

Ankesh Saha  added the comment:

Hi,

I have ran the code with with _generate_next_value_ method at the bottom of the 
class and didn't run into any exceptions. Please refer my python file. Please 
let me know if I am missing something.

from enum import Enum, auto

class E(Enum):
A = auto()
B = auto()
def _generate_next_value_(name, *args):
return name

for l in (E):
print(l.name)
print(l.value)

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Added file: https://bugs.python.org/file49014/Issue40025.PNG

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Jonathan Hsu]

Jonathan Hsu  added the comment:

Thank you for the explanation.

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:

Jonathan Hsu, you are correct -- and "don't do that" was my initial response; 
but Enum takes many pains to make sure the user doesn't shoot themselves in the 
foot, so raising a TypeError is appropriate instead of silently doing the wrong 
thing.

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Jonathan Hsu]

Jonathan Hsu  added the comment:

While the current behavior may be initially unexpected, it does match the way 
that python normally behaves when defining class variables. For example, the 
following class will throw an exception because the function number_two() is 
called before it is defined:


class Numbers:
one = 1
two = number_two()

def number_two():
return 2

# NameError: name 'number_two' is not defined


However, this version is fine:


class Numbers:
one = 1

def number_two():
return 2

two = number_two()

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Onstott]

Change by Ethan Onstott :


--
keywords: +patch
nosy: +Ethan Onstott
nosy_count: 5.0 -> 6.0
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stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/19098

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Ethan Furman]

Ethan Furman  added the comment:

Immediate solution is to raise an exception if `_generate_next_value_` is 
defined after members.

Possible future solution is to save all member definitions until after class is 
defined.

The exception-raising solution would require a check in `_EnumDict` where 
`_generate_next_value_` is saved -- if any members already exist, raise.

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assignee: docs@python -> ethan.furman
keywords: +easy
stage:  -> needs patch
versions: +Python 3.8, Python 3.9

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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Luis E.]

Change by Luis E. :


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[issue40025] enum: _generate_next_value_ is not called if its definition occurs after calls to auto()

[Luis E.]

New submission from Luis E. :

I ran into this issue when attempting to add a custom _generate_next_value_ 
method to an existing Enum. Adding the method definition to the bottom of the 
class causes it to not be called at all:

from enum import Enum, auto

class E(Enum):
A = auto()
B = auto()
def _generate_next_value_(name, *args):
return name


E.B.value  # Returns 2, E._generate_next_value_ is not called

class F(Enum):
def _generate_next_value_(name, *args):
return name
A = auto()
B = auto()


F.B.value  # Returns 'B', as intended


I do not believe that the order of method/attribute definition should affect 
the behavior of the class, or at least it should be mentioned in the 
documentation.

--
assignee: docs@python
components: Documentation, Library (Lib)
messages: 364665
nosy: docs@python, edd07
priority: normal
severity: normal
status: open
title: enum: _generate_next_value_ is not called if its definition occurs after 
calls to auto()
type: behavior
versions: Python 3.7

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