PEP 378 uses replace where translate may work better

[samwyse]

BPO	10178
Nosy	@rhettinger, @ncoghlan, @ericvsmith, @bitdancer

^{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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assignee = None
closed_at = <Date 2010-10-23.21:55:08.782>
created_at = <Date 2010-10-23.14:05:24.043>
labels = ['type-bug', 'docs']
title = 'PEP 378 uses replace where translate may work better'
updated_at = <Date 2010-10-23.21:55:08.781>
user = 'https://bugs.python.org/samwyse'

bugs.python.org fields:

activity = <Date 2010-10-23.21:55:08.781>
actor = 'rhettinger'
assignee = 'docs@python'
closed = True
closed_date = <Date 2010-10-23.21:55:08.782>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2010-10-23.14:05:24.043>
creator = 'samwyse'
dependencies = []
files = []
hgrepos = []
issue_num = 10178
keywords = []
message_count = 4.0
messages = ['119427', '119429', '119445', '119477']
nosy_count = 6.0
nosy_names = ['rhettinger', 'ncoghlan', 'eric.smith', 'samwyse', 'r.david.murray', 'docs@python']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue10178'
versions = ['Python 3.1', 'Python 2.7']

[samwyse]

PEP-378 states;

format(n, "6,f").replace(",", "X").replace(".", ",").replace("X", ".")

This is complex and relatively slow. A better technique, which IMHO the proposal should high-lighted, would be:

  swap_commas_and_periods = bytes.maketrans(b',.', b'.,')
  format(n, "6,f").translate(swap_commas_and_periods)

While performing the maketrans each time a string is formatted is slower than the triple replace, calling it once and caching the result is faster. I have tested with with the 3.1 interpreter; example timings follow.

>>> Timer("""
  '1,234,567.89'.replace(',', 'X').replace('.', ',').replace('X', '.')
""").timeit()
3.0645400462908015

>>> Timer("""
  '1,234,567.89'.translate(swap_commas_and_periods)
""", """
  swap_commas_and_periods = bytes.maketrans(b',.', b'.,')
""").timeit()
2.276630409730846


>>> Timer("""
  '1,234,567.89'.translate(bytes.maketrans(b',.', b'.,'))
""").timeit()
3.760715677551161

[bitdancer]

The text in question is talking about 'replace' as a general mechanism for 'fixing' the separator character, and as such I don't think introducing translate would enhance the exposition. I suppose it could be added in a footnote.

[samwyse]

The text in question is also talking about the problems with using 'replace' to swap pairs of characters, so a better, alternate, process would be valuable, especially for anyone unaware of the translate method.

[rhettinger]

Sorry, the text needs to stand as-is.
It is supposed to be a hint of what can be done,
nothing more.

The technique of t=x; x=y; y=t is somewhat basic
and has general applicability -- there's nothing
"complex" about it. Also, for short strings
such as the one in the example, the translate
approach is slower unless the used in a loop
where the translation table is already built.

BTW, the PEP itself is not primary documentation
for users. It is meant to document the design
discussion only.

Feel free to post your recipe on ASPN or on
the newsgroup.