str.strip: argument maxstrip
I often find myself in a need of stripping only a specified amount of characters from a string (most commonly a single character). I have been implementing this in an ad-hoc manner, which is quite inelegant:
def rstrip1(txt, chars): if txt is None: return None elif any(txt.endswith(c) for c in chars): return txt[:-1] else: return txt
I would appreciate if str.split, str.lstrip, str.rsplit functions had a `maxstrip` argument, similar to the `maxsplit` argument of `str.split`, which would specify the maximum count of characters to be removed from the string. In case of `str.split` this maximum would apply to each side of the string separately.
Am I the only one who is surprised such functionality is not implemented already??
On Tue, May 19, 2020 at 9:00 AM computermaster360 . < computermaster360@gmail.com> wrote:
I often find myself in a need of stripping only a specified amount of characters from a string (most commonly a single character). I have been implementing this in an ad-hoc manner, which is quite inelegant:
def rstrip1(txt, chars): if txt is None: return None elif any(txt.endswith(c) for c in chars): return txt[:-1] else: return txt
I would appreciate if str.split, str.lstrip, str.rsplit functions had a `maxstrip` argument, similar to the `maxsplit` argument of `str.split`, which would specify the maximum count of characters to be removed from the string. In case of `str.split` this maximum would apply to each side of the string separately.
Do you mean "str.strip, str.lstrip, str.rstrip"?
To strip at most 1 character from the end: txt[:-1] + txt[-1:].rstrip(chars) To strip at most N characters: txt[:-N] + txt[-N:].rstrip(chars) Given that, I think yours is too much of a niche case to change anything in Python Rob Cliffe
On 19/05/2020 12:44, computermaster360 . wrote:
I often find myself in a need of stripping only a specified amount of characters from a string (most commonly a single character). I have been implementing this in an ad-hoc manner, which is quite inelegant:
def rstrip1(txt, chars): if txt is None: return None elif any(txt.endswith(c) for c in chars): return txt[:-1] else: return txt
I would appreciate if str.split, str.lstrip, str.rsplit functions had a `maxstrip` argument, similar to the `maxsplit` argument of `str.split`, which would specify the maximum count of characters to be removed from the string. In case of `str.split` this maximum would apply to each side of the string separately.
Am I the only one who is surprised such functionality is not implemented already?? _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/NSUFM4... Code of Conduct: http://python.org/psf/codeofconduct/
I think this would be useful, and doesn't break any backward compatibility. I would have use for it from time to time myself.
On Tue, May 19, 2020 at 9:01 AM computermaster360 . < computermaster360@gmail.com> wrote:
I often find myself in a need of stripping only a specified amount of characters from a string (most commonly a single character). I have been implementing this in an ad-hoc manner, which is quite inelegant:
def rstrip1(txt, chars): if txt is None: return None elif any(txt.endswith(c) for c in chars): return txt[:-1] else: return txt
I would appreciate if str.split, str.lstrip, str.rsplit functions had a `maxstrip` argument, similar to the `maxsplit` argument of `str.split`, which would specify the maximum count of characters to be removed from the string. In case of `str.split` this maximum would apply to each side of the string separately.
Am I the only one who is surprised such functionality is not implemented already?? _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/NSUFM4... Code of Conduct: http://python.org/psf/codeofconduct/
David (or somebody else) could you give us some, as real as possible, examples? This will strengthen the case for it!
I am confident they exist and are pretty plentiful but I myself am coming up blank thinking about it for a few minutes and documenting them would be good for discussion.
On Tue, 19 May 2020 at 18:59, David Mertz mertz@gnosis.cx wrote:
I think this would be useful, and doesn't break any backward compatibility. I would have use for it from time to time myself.
On Tue, May 19, 2020 at 9:01 AM computermaster360 . < computermaster360@gmail.com> wrote:
I often find myself in a need of stripping only a specified amount of characters from a string (most commonly a single character). I have been implementing this in an ad-hoc manner, which is quite inelegant:
def rstrip1(txt, chars): if txt is None: return None elif any(txt.endswith(c) for c in chars): return txt[:-1] else: return txt
I would appreciate if str.split, str.lstrip, str.rsplit functions had a `maxstrip` argument, similar to the `maxsplit` argument of `str.split`, which would specify the maximum count of characters to be removed from the string. In case of `str.split` this maximum would apply to each side of the string separately.
Am I the only one who is surprised such functionality is not implemented already?? _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/NSUFM4... Code of Conduct: http://python.org/psf/codeofconduct/
-- The dead increasingly dominate and strangle both the living and the not-yet born. Vampiric capital and undead corporate persons abuse the lives and control the thoughts of homo faber. Ideas, once born, become abortifacients against new conceptions. _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/ZRR3ME... Code of Conduct: http://python.org/psf/codeofconduct/
On Tue, May 19, 2020 at 8:10 PM Henk-Jaap Wagenaar < wagenaarhenkjaap@gmail.com> wrote:
David (or somebody else) could you give us some, as real as possible, examples? This will strengthen the case for it!
I am confident they exist and are pretty plentiful but I myself am coming up blank thinking about it for a few minutes and documenting them would be good for discussion.
I agree, I can't think of use cases for this.
Another way to solve this:
```
re.sub(r"[123]{,4}$", "", "abc123123")
'abc12' ```
I did a couple greps of my git/ directory to see if I found examples. Given that there are a couple ways one might achieve the effect now, I don't necessarily find everything. But here's something in software I did not write myself.
This is from ./JohnTheRipper/run/sspr2john.py. I found another example in a different file, but looking at it I'm pretty sure it is actually a potential bug (it has a comment similar to "Is this safe?" which I won't bother showing.
elif fmt == "PBKDF2_SHA256": h = base64.b64encode(base64.b64decode(text)[:32]) # a terrible hack follows, use "adapted base64" alphabet (using . instead of + and with no padding) h = h.rstrip("=").replace("+", ".") salt = base64.b64encode(salt) salt = salt.rstrip("=").replace("+", ".")
We actually know that base64 code should only produce at most 2 '='s as padding. In this instance, the encoding comes immediately before the stripping. However, perhaps some code would pass the encoded string and you wouldn't be as confident locally that extra '='s hadn't snuck in.
If it existed, I think these lines would be good candidates for 'maxstrip'.
On Tue, May 19, 2020 at 2:07 PM Henk-Jaap Wagenaar < wagenaarhenkjaap@gmail.com> wrote:
David (or somebody else) could you give us some, as real as possible, examples? This will strengthen the case for it!
I am confident they exist and are pretty plentiful but I myself am coming up blank thinking about it for a few minutes and documenting them would be good for discussion.
On Tue, May 19, 2020 at 8:49 PM David Mertz mertz@gnosis.cx wrote:
elif fmt == "PBKDF2_SHA256": h = base64.b64encode(base64.b64decode(text)[:32]) # a terrible hack follows, use "adapted base64" alphabet(using . instead of + and with no padding) h = h.rstrip("=").replace("+", ".") salt = base64.b64encode(salt) salt = salt.rstrip("=").replace("+", ".")
We actually know that base64 code should only produce at most 2 '='s as padding. In this instance, the encoding comes immediately before the stripping. However, perhaps some code would pass the encoded string and you wouldn't be as confident locally that extra '='s hadn't snuck in.
If it existed, I think these lines would be good candidates for 'maxstrip'.
Not a very strong ending 🤣
I may be misunderstanding, but it sounds like = is not acceptable in the final result, so it's not enough to remove only 2 of 4 ='s. You want to make sure nothing messed up your string. So if the code existed, what you'd want is:
``` assert salt.count("=") <= 2 salt = salt.rstrip("=", "") assert "=" not in salt ```
I may be misunderstanding, but it sounds like = is not acceptable in the final result, so it's not enough to remove only 2 of 4 ='s. You want to make sure nothing messed up your string. So if the code existed, what you'd want is:
assert salt.count("=") <= 2 salt = salt.rstrip("=", "") assert "=" not in salt
I think the code I'd want, if the new parameter existed, would be:
salt = salt.rstrip("=", maxstrip=2) assert not salt.endswith("=")
That feels *slightly* better than your version. This version would allow there to be '=' in the middle of the string (albeit, such would not be base64, but some other kind of thing). Obviously, I've managed to program Python without this switch for 20+ years. But I think I'd use it occasionally.
Currently, I'd probably program my intention like this. Let's assume this is something where the '=' is allowed in the middle.
if salt.endswith("=="): salt = salt[-2:] elif salt.endswith("="): salt = salt[-1:] assert not salt.endswith("=")
The version you suggested, as mentioned, would not handle a format where '=' was permitted in the middle.
On Tue, May 19, 2020 at 9:43 PM David Mertz mertz@gnosis.cx wrote:
Currently, I'd probably program my intention like this. Let's assume this is something where the '=' is allowed in the middle.
Getting increasingly hypothetical...
Anyway, you could write:
``` assert not salt.endswith("=" * 3) # at most 2 ='s allowed salt = salt.rstrip("=") ```
On Tue, May 19, 2020 at 3:50 PM Alex Hall alex.mojaki@gmail.com wrote:
Anyway, you could write: assert not salt.endswith("=" * 3) # at most 2 ='s allowed salt = salt.rstrip("=")
Yes, I *could* write that. It feels a bit more cryptic than the version I mentioned. But it's fine.
On 19May2020 15:43, David Mertz mertz@gnosis.cx wrote:
I may be misunderstanding, but it sounds like = is not acceptable in the final result, so it's not enough to remove only 2 of 4 ='s. You want to make sure nothing messed up your string. So if the code existed, what you'd want is:
assert salt.count("=") <= 2 salt = salt.rstrip("=", "") assert "=" not in saltI think the code I'd want, if the new parameter existed, would be:
salt = salt.rstrip("=", maxstrip=2) assert not salt.endswith("=")
Reiterating the Python 3.9 suggestion, what about:
salt2 = salt.cutsuffix(('==', '='))
I appreciate this isn't as _general_ as a maxstrip param, but it seems to neatly address the single use case you've found.
Cheers, Cameron Simpson cs@cskk.id.au
On Tue, May 19, 2020 at 8:43 PM Cameron Simpson cs@cskk.id.au wrote:
salt = salt.rstrip("=", maxstrip=2) assert not salt.endswith("=")
Reiterating the Python 3.9 suggestion, what about:
salt2 = salt.cutsuffix(('==', '='))
You'd have to call cutsuffix twice. Valid base64 might end in one, two, or zero '=', but the result wants to have none. Actually, no... even two calls won't always do the right thing. I'm happy to have the new method .cutsuffix() , but I don't think it addresses this case.
I was going to suggest an example about dynamically built path components that may or may not end in '/'. However:
A. I couldn't find an example in my own code easily, even though I know I've fiddled with that B. Someone would scold me for playing with strings rather than using Pathlib (they'd be right, I realize... but I am sinful in my quick-script ways).
I'll let someone else, like the OP, advance the case more. I kinda like it, but it's not a big deal for me.
On Wed, May 20, 2020 at 2:46 AM Cameron Simpson cs@cskk.id.au wrote:
On 19May2020 15:43, David Mertz mertz@gnosis.cx wrote: Reiterating the Python 3.9 suggestion, what about:
salt2 = salt.cutsuffix(('==', '='))
Tuple arguments were rejected in the PEP.
``` Python 3.10.0a0 (heads/master:bac170cd93, May 20 2020, 12:20:34) [Clang 10.0.0 (clang-1000.11.45.5)] on darwin Type "help", "copyright", "credits" or "license" for more information.
'abc=='.removesuffix('=')
'abc='
'abc=='.removesuffix(('==', '='))
Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: removesuffix() argument must be str, not tuple ```
But if the API was there, I agree this would work, not sure what David is saying about needing to call twice. On the other hand, this example demonstrates well how a tuple is potentially confusing. What happens if you call `'abc=='.removesuffix(('=', '=='))`?
On Wed, 20 May 2020 at 11:34, Alex Hall alex.mojaki@gmail.com wrote:
On Wed, May 20, 2020 at 2:46 AM Cameron Simpson cs@cskk.id.au wrote:
On 19May2020 15:43, David Mertz mertz@gnosis.cx wrote: Reiterating the Python 3.9 suggestion, what about:
salt2 = salt.cutsuffix(('==', '='))
But if the API was there, I agree this would work, not sure what David is saying about needing to call twice. On the other hand, this example demonstrates well how a tuple is potentially confusing. What happens if you call `'abc=='.removesuffix(('=', '=='))`?
I assume that was the practical "how to do it now":
foobar.removesuffix('=').removesuffix('=')
would work right now in 3.9 (I think).
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On Wed, May 20, 2020 at 12:44 PM Henk-Jaap Wagenaar < wagenaarhenkjaap@gmail.com> wrote:
foobar.removesuffix('=').removesuffix('=')
would work right now in 3.9 (I think). http://python.org/psf/codeofconduct/
Can confirm:
```
'abc==='.removesuffix('=').removesuffix('=')
'abc='
'abc=='.removesuffix('=').removesuffix('=')
'abc'
'abc='.removesuffix('=').removesuffix('=')
'abc'
'abc'.removesuffix('=').removesuffix('=')
'abc' ```
On Tue, May 19, 2020 at 2:58 PM computermaster360 . < computermaster360@gmail.com> wrote:
I often find myself in a need of stripping only a specified amount of characters from a string (most commonly a single character). I have been implementing this in an ad-hoc manner, which is quite inelegant:
def rstrip1(txt, chars): if txt is None: return None elif any(txt.endswith(c) for c in chars): return txt[:-1] else: return txt
If you want to remove at most one instance of one specific character, then I believe in Python 3.9 you will be able to write `txt.removesuffix(char)` - see https://www.python.org/dev/peps/pep-0616/.
For removing one of several possible characters, the potential API for that has currently been rejected: https://www.python.org/dev/peps/pep-0616/#accepting-a-tuple-of-affixes
participants (7)
-
Alex Hall -
Cameron Simpson -
computermaster360 . -
David Mertz -
Henk-Jaap Wagenaar -
Jonathan Goble -
Rob Cliffe