Issue 5621: Add description of special case to "Assignment statements" section (original) (raw)

Created on 2009-03-31 14:45 by jjposner, last changed 2022-04-11 14:56 by admin. This issue is now closed.

Messages (6)

msg84790 - (view)

Author: John Posner (jjposner) *

Date: 2009-03-31 14:45

The subsection "Augmented assignment statements" includes a note on this special case:

a.x += 1

But the parent section "Assignment statements" does not include such a note, even though it's essentially the same situation. I suggest replacing the bulleted paragraph that begins "If the target is an attribute reference" with the following:

• If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, TypeError is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily AttributeError).

  If the object is a class instance and the attribute reference occurs on both sides of the assignment operator; for example::

  a.x = a.x + 1

  ... in the RHS expression, a.x is evaluated with getattr(), which can access either an instance attribute or (if no instance attribute exists) a class attribute. The LHS target a.x is assigned with setattr(), which always accesses an instance attribute, creating it if necessary. Thus, the two occurrences of a.x do not necessarily refer to the same variable. If the RHS expression refers to a class attribute, the LHS creates a new instance attribute as the target of the assignment. (This description does not necessarily apply to attributes defined with property(), which are accessed with user-defined functions instead of getattr() and setattr()).

  See section "Augmented assignment statements" for a similar note on attribute references.

msg84800 - (view)

Author: Georg Brandl (georg.brandl) *

Date: 2009-03-31 15:33

I'm not sure it belongs there, since normal assignments do not say anything about the RHS.

msg84805 - (view)

Author: John Posner (jjposner) *

Date: 2009-03-31 15:46

The "Assignment statements" section does talk about the RHS -- but in a subtle way:

For targets which are attribute references, the initial value is retrieved with a getattr()

The retrieving of the initial value (maybe "current value" would be better) occurs on the RHS of the assignment statement.

msg84822 - (view)

Author: Georg Brandl (georg.brandl) *

Date: 2009-03-31 16:15

Yes, it says that getattr() is used in the section about augassign; but there the note you refer to is already present.

msg92420 - (view)

Author: John Posner (jjposner) *

Date: 2009-09-08 15:39

George, here is a patch file for this bug. It modifies file doc/reference/simple_stmts.rst

Please let me know if this was the wrong way to submit the patch file.

msg92677 - (view)

Author: Georg Brandl (georg.brandl) *

Date: 2009-09-16 10:13

The patch submission was correct.

I edited the patch a bit, moved the example from augmented assignment to the regular assignment and added a link from there to the new section.

Committed in r74822.

History

Date

User

Action

Args

2022-04-11 14:56:47

admin

set

github: 49871

2009-09-16 10:13:15

georg.brandl

set

status: open -> closed
resolution: works for me -> accepted
messages: +

2009-09-08 15:39:23

jjposner

set

files: + assignment_statement.diff
keywords: + patch
status: pending -> open
messages: +

2009-03-31 16:15:33

georg.brandl

set

status: open -> pending
resolution: works for me
messages: +

2009-03-31 15:46:00

jjposner

set

messages: +

2009-03-31 15:33:22

georg.brandl

set

messages: +

2009-03-31 14:45:05

jjposner

create