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surprised by default list parameter #48348

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trott mannequin opened this issue Oct 10, 2008 · 4 comments
Closed

surprised by default list parameter #48348

trott mannequin opened this issue Oct 10, 2008 · 4 comments
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build The build process and cross-build type-bug An unexpected behavior, bug, or error

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@trott
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Mannequin

trott mannequin commented Oct 10, 2008

BPO 4098
Nosy @amauryfa
Files
  • p6.py: just run the example file "p6.py"
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
closed_at = <Date 2008-10-10.11:21:21.675>
created_at = <Date 2008-10-10.09:23:43.974>
labels = ['type-bug', 'invalid', 'build']
title = 'surprised by default list parameter'
updated_at = <Date 2008-10-10.11:21:21.634>
user = 'https://bugs.python.org/trott'

    bugs.python.org fields:

    activity = <Date 2008-10-10.11:21:21.634>
actor = 'amaury.forgeotdarc'
assignee = 'none'
closed = True
closed_date = <Date 2008-10-10.11:21:21.675>
closer = 'amaury.forgeotdarc'
components = ['Build']
creation = <Date 2008-10-10.09:23:43.974>
creator = 'trott'
dependencies = []
files = ['11761']
hgrepos = []
issue_num = 4098
keywords = []
message_count = 4.0
messages = ['74624', '74626', '74627', '74628']
nosy_count = 3.0
nosy_names = ['amaury.forgeotdarc', 'eckhardt', 'trott']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue4098'
versions = ['Python 2.6', 'Python 2.5', 'Python 2.5.3']
    @trott
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    Mannequin Author

    trott mannequin commented Oct 10, 2008

    The attached file produced the following result:

    trott$ python2.5 p6.py
    []
    [1234]

    I expected both times the empty list since the print
    statement prints just a defaulted parameter ("e") which
    was defaulted to an empty list literal ("[]") and never set
    by supplying an actual class parameter.

    The object property "entries" appears to be shared by
    both instances. Why?

    @trott trott mannequin added build The build process and cross-build type-bug An unexpected behavior, bug, or error labels Oct 10, 2008
    @eckhardt
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    Mannequin

    eckhardt mannequin commented Oct 10, 2008

    http://effbot.org/pyfaq/why-are-default-values-shared-between-objects.htm

    The same recently cropped up on the users mailinglist under the topic
    "default value in __init__".

    @trott
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    Mannequin Author

    trott mannequin commented Oct 10, 2008

    OK. I've read the Python Reference Manual 7.6 "Function Definition".
    It "explains" why this happens.

    BUT: Why can`t the compiler make a copy of the default object, if
    it's mutable (e.g. a list)? This can't be that difficult.

    @amauryfa
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    Yes, the default value expression could be evaluated each time the
    function is called without the parameter. But it would be another language.
    Default values are computed once, and this is a Python feature.

    It can be useful, for example to cache computations, or to achieve
    "early binding" in nested functions, as explained in the suggested FAQ.

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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