[issue28612] str.translate needs a mapping example

[Cheryl Sabella]

Cheryl Sabella  added the comment:

IDLE just added similar functionality to pyparse (issue 32940) using:

class ParseMap(dict):
def __missing__(self, key):
return 120 # ord('x')

# Map all ascii to 120 to avoid __missing__ call, then replace some.
trans = ParseMap.fromkeys(range(128), 120)
trans.update((ord(c), ord('(')) for c in "({[")  # open brackets => '(';
trans.update((ord(c), ord(')')) for c in ")}]")  # close brackets => ')'.
trans.update((ord(c), ord(c)) for c in "\"'\\\n#") # Keep these.

code = code.translate(trans)

Of course, all that is probably too much for a docs example, but it uses a 
mapping without the side effect of defaultdict.  I wonder if defining the dict 
subclass with __missing__ and then the example of keeping only lowercase 
letters would work for the docs?

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nosy: +csabella

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[issue28612] str.translate needs a mapping example

[Christopher Barker]

Christopher Barker added the comment:

This all came out of a thread on python-ideas, starting here:

https://mail.python.org/pipermail/python-ideas/2016-October/043284.html

the thread kind of petered out, but it seems there was a kinda-sorta
consensus that we didn't need any new string methods, but rather same notes
in the docs on how to to use .translate() to remove "all but these" was in
order.

And the defaultdict method was proposed as the easiest / most pythonic.

As it happens, I did't live the fact hat defaultdict will build up a
big(ish) dict of Nones for no reason, and thus suggested a NoneDict option:

class NoneDict(dict):
"""
Dictionary implementation that always returns None when a key is not in
the dict,
rather than raising a KeyError
"""
def __getitem__(self, key):
try:
val = dict.__getitem__(self, key)
except KeyError:
val = None
return val

Though maybe that's a bit much for the docs.

However, in short:

either the defaultdict approach is siple and pythonic enough to be in teh
docs, or we SHOULD add something new to the string object.

(or maybe someone has another nifty pythonic way to do this with the stdlib
that's better than defaultdict?)

-CHB

On Fri, Dec 30, 2016 at 12:18 PM, Gaurav Tatke 
wrote:

>
> Gaurav Tatke added the comment:
>
> Should a user be suggested to use str.translate() for the use case where
> user only wants to keep certain characters and strip off everything else?
>
> --
>
> ___
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> 
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-- 

Christopher Barker, Ph.D.
Oceanographer

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[issue28612] str.translate needs a mapping example

[Gaurav Tatke]

Gaurav Tatke added the comment:

Should a user be suggested to use str.translate() for the use case where user 
only wants to keep certain characters and strip off everything else?

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[issue28612] str.translate needs a mapping example

[Gaurav Tatke]

Gaurav Tatke added the comment:

Hi,

Pardon my ignorance, I am new to this but have below queries/thoughts -

1. Why would we say that adding new keys during lookup phase is an unpleasant 
side-effect? From what I understood by docs, one of the main reasons to use 
defaultdicts is to be able to insert a missing key and give a default value to 
it. 'defaultdict' doc itself suggest that doing this is cleaner and faster than 
using dict.setdefault().

2. I believe defaultdict perfectly fits in this context of creating translation 
table for str.translate(). Even if we have very large string containing all 
characters from 4-5 languages, our defaultdict will still be comparatively 
small. It is easier to create a translation table using defaultdict when we 
have to strip most characters out of a string as in the example requested in 
the issue. Creating a translation table using str.maketrans() or by user 
defined function is tricky in this use case.

3. If argument for using defaultdict in this context is not convincing, shall I 
just give a patch, giving example of str.translate() using str.maketranse()?

Regards,

--

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[issue28612] str.translate needs a mapping example

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

If the side effect of defaultdict is unpleasant, the correct way is combining 
the translation mapping with the custom mapping by ChainMap. But this example 
is too complex for the documentation of str.translate(). On other side, it is 
trivial for more experience users and don't need special mentioning.

I think other resources (ActiveState Code Reciepes [1] or books) are better 
places for this example.

[1] http://code.activestate.com/recipes/popular/

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[issue28612] str.translate needs a mapping example

[Raymond Hettinger]

Raymond Hettinger added the comment:

I like the idea of adding a mapping example but don't want to encourage use of 
defaultdict in contexts like this one.  A defaultdict usefully specifies a 
default but has the unpleasant side-effect of altering the dictionary (adding 
new keys) during the look-up phase.  This has bitten a lot of people (including 
famous ones like Peter Norvig).

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[issue28612] str.translate needs a mapping example

[Gaurav Tatke]

Gaurav Tatke added the comment:

Hi,

I am new to Python and want to contribute. I am attaching a patch having 
required example of using defaultdict with translate. Please let me know if 
anything needs to be changed. I have tested the example and also the html doc 
in my local.

Regards,
Gaurav

--
keywords: +patch
nosy: +Gaurav Tatke
Added file: http://bugs.python.org/file46072/translateexample_issue28612.patch

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[issue28612] str.translate needs a mapping example

[Chris Barker]

Chris Barker added the comment:

Agreed:

the custom dict type would be nice for a recipe or blog post or...

but not for the docs.

I'll note that the other trick to this recipe is that you need to know to use 
lambda to make a "None factory" for defaultdict -- though maybe that's a ToDo 
for the defaultdict docs...

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[issue28612] str.translate needs a mapping example

[Jim Jewett]

Jim Jewett added the comment:

https://mail.python.org/pipermail/python-ideas/2016-November/043539.html by 
Chris Barker points out that a custom object (which doesn't ever store the 
missing "keys") may be better still... though I'm not sure it is better enough 
to complicate the docs.

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[issue28612] str.translate needs a mapping example

[Jim Jewett]

New submission from Jim Jewett:

One commonly needed string transformation is stripping out certain characters 
(or only keeping certain characters).  This is common enough that it might be 
worth a dedicated method, except, that, as Stephen J. Turnbull wrote in 
https://mail.python.org/pipermail/python-ideas/2016-November/043501.html

"""
So really translate with defaultdict is a specialized loop that
marries an algorithmic body (which could do things like look up the
original script or other character properties to decide on the
replacement for the generic case) with a (usually "small") table of
exceptions.  That seems like inspired design to me.
"""

Alas, while inspired, it isn't obvious to someone who isn't yet used to the 
power of python custom classes.

The documentation (such as 
https://docs.python.org/3/library/stdtypes.html?highlight=translate#str.translate
 ) should include such an example.

One possible example would be a defaultdict that says to discard any characters 
except lower case ASCII lettersI.

--
assignee: docs@python
components: Documentation
keywords: easy
messages: 280061
nosy: Jim.Jewett, docs@python
priority: normal
severity: normal
stage: needs patch
status: open
title: str.translate needs a mapping example
type: enhancement
versions: Python 3.5, Python 3.6, Python 3.7

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