gh-55688: Add note about ending backslashes for raw strings #94768

slateny · 2022-07-12T04:42:10Z

https://docs.python.org/3/tutorial/introduction.html

Co-authored-by: R. David Murray rdmurray@bitdance.com

Issue: Add discussion of trailing backslash in raw string to tutorial #55688

Issue: Add discussion of trailing backslash in raw string to tutorial #55688

Doc/tutorial/introduction.rst

eendebakpt · 2022-10-22T22:02:41Z

Doc/tutorial/introduction.rst

@@ -189,6 +189,29 @@ the first quote::
   >>> print(r'C:\some\name')  # note the r before the quote
   C:\some\name

+There is one subtle aspect to raw strings: a raw string may not end in an odd


The text of the addition is fine. But I am wondering whether such a subtle point should be part of the introduction.rst. Perhaps keep only the first line and refer to a location with details?

(not sure what would be a better location)

It's not strictly a part of this file, see https://docs.python.org/3/reference/lexical_analysis.html:

... even a raw string cannot end in an odd number of backslashes ...

The text you added in this PR will end up here: https://docs.python.org/3/tutorial/introduction.html#strings right?

It is an informal introduction, so I would write something like:

There is one subtle aspect to raw strings: a raw string may not end in an odd
number of \ characters. For details see String and Bytes literals

But the text added is correct, so it is mostly a matter of style.

Re-reading this, I realize that my comment above doesn't actually address your original concern on whether this is too subtle of a point for the tutorial introduction - IMO, since the section above does talk about escaping quotes as well as raw strings, I think it's somewhat natural to at least mention the case with them combined.

As for how brief a mention, the current phrasing can definitely be made more brief. I think also that just one of the workarounds can be left in, while the other two examples put elsewhere, along with the wording specific to Windows paths. If the section would introduce the problem of raw strings not ending in an odd number of slashes, I think it should also suggest some sort of solution to it, so I would prefer keeping one example in.

As for moving the rest of the information, I don't think the reference is the most appropriate place, maybe instead in some FAQ entry or a howto page?

An FAQ entry sounds reasonable. Maybe we do something like There is one subtle aspect to raw strings: a raw string may not end in an odd number of \ characters; see [the FAQ entry] for workarounds?

hauntsaninja

I agree with eendebakpt that this seems like too subtle a point to spend this much space discussing in the tutorial, so I think we should try to find another home for this.

Maybe an alternate approach would be to add some special casing to give users a more helpful error message in this case?

…ntry

hauntsaninja

Thanks, this looks broadly good to me.

My favourite workaround for this would actually be to use (implicit) concatenation. So:

r'C:\this\will\work' '\\'

or

r'C:\this\will\work' + '\\'

I think this is probably less error prone than the strip approach (e.g. you probably want to use rstrip, and what if you have more trailing spaces?), so maybe we swap that workaround for concatenation?

hauntsaninja · 2022-12-25T20:55:30Z

Oh one other thing that is sort of confusing that we could clarify here is that in r'asdf\'asdf' the backslash only escapes the quote for the purposes of tokenisation; the backslash still appears in the final string. I'm not sure the best way to phrase this, but if you think of something good we could mention it!

…n r-string

slateny · 2022-12-25T23:00:23Z

Doc/tutorial/introduction.rst

@@ -189,6 +189,16 @@ the first quote::
   >>> print(r'C:\some\name')  # note the r before the quote
   C:\some\name

+Note that escaping quotes in raw strings will keep the backslash::


Maybe to be explicit, it could be something like

Suggested change

Note that escaping quotes in raw strings will keep the backslash::

Note that, unlike the non–raw string case, escaping quotes in raw strings will keep the backslash::

Thanks for making the change! This looks good, but let's move it to the FAQ section?

My reasoning is based on the description of raw strings it's not really surprising that the backslash is preserved. It's only really surprising in the context of the end quote situation, where the backslash "escapes" the quote for the tokeniser, but not for the actual value.

That is, if you read:

A raw string ending with an odd number of backslashes will escape the string's quote

you may come away with the impression that raw strings are raw except for quotes or something. I guess a concrete phrasing would be adding the following to the end of the FAQ entry:

Note that while a backslash will "escape" a quote for the purposes of determining where the raw string ends, there are no escape sequences that affect the interpretation of the value of the raw string. That is, the backslash remains present in the value of the raw string:: >>> r'backslash\'preserved' "backslash\\'preserved"

Okay, there's prior art. Here's the relevant words from Lexical Analysis:

Even in a raw literal, quotes can be escaped with a backslash, but the backslash remains in the result; for example, r"\"" is a valid string literal consisting of two characters: a backslash and a double quote; r"\" is not a valid string literal (even a raw string cannot end in an odd number of backslashes). Specifically, a raw literal cannot end in a single backslash (since the backslash would escape the following quote character). Note also that a single backslash followed by a newline is interpreted as those two characters as part of the literal, not as a line continuation.

Hmm, I'm not too certain what the tokenization / determining where the raw string ends means too well, but I've commited the changes (and added a link to the reference) for the time being.

hauntsaninja

Thanks, this looks good to me cc @JelleZijlstra

miss-islington · 2022-12-28T05:30:45Z

Thanks @slateny for the PR, and @hauntsaninja for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.11.
🐍🍒⛏🤖

miss-islington · 2022-12-28T05:30:52Z

Sorry @slateny and @hauntsaninja, I had trouble checking out the 3.11 backport branch.
Please retry by removing and re-adding the "needs backport to 3.11" label.
Alternatively, you can backport using cherry_picker on the command line.
cherry_picker b95b1b3b25b0a93a22c7d58ac5bd5870e62070a8 3.11

bedevere-bot · 2022-12-28T05:31:36Z

GH-100570 is a backport of this pull request to the 3.10 branch.

…thonGH-94768) (cherry picked from commit b95b1b3) Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com> Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

miss-islington · 2022-12-28T05:38:43Z

Thanks @slateny for the PR, and @hauntsaninja for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

bedevere-bot · 2022-12-28T05:38:50Z

GH-100571 is a backport of this pull request to the 3.11 branch.

…thonGH-94768) (cherry picked from commit b95b1b3) Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com> Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

(cherry picked from commit b95b1b3) Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com> Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

Add note about ending backslashes for raw strings

0dc3888

bedevere-bot added awaiting review docs Documentation in the Doc dir labels Jul 12, 2022

slateny added the skip news label Jul 12, 2022

slateny requested a review from bitdancer October 13, 2022 05:01

eendebakpt reviewed Oct 22, 2022

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Be more specific with platform

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eendebakpt approved these changes Oct 24, 2022

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bedevere-bot added awaiting core review and removed awaiting review labels Oct 24, 2022

hauntsaninja reviewed Dec 23, 2022

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hauntsaninja mentioned this pull request Dec 23, 2022

Improve error message for unterminated strings with escapes #100445

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Move information on raw string and backslashes from tutorial to FAQ e…

9e3d190

…ntry

bedevere-bot mentioned this pull request Dec 25, 2022

Add discussion of trailing backslash in raw string to tutorial #55688

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hauntsaninja reviewed Dec 25, 2022

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Use string concatenation over strip; add note on backslashed quotes i…

efad634

…n r-string

slateny commented Dec 25, 2022

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Reword text and move to faq

b8e6594

hauntsaninja added needs backport to 3.10 only security fixes needs backport to 3.11 only security fixes labels Dec 28, 2022

hauntsaninja approved these changes Dec 28, 2022

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bedevere-bot added awaiting merge and removed awaiting core review labels Dec 28, 2022

JelleZijlstra approved these changes Dec 28, 2022

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improve wording slightly

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hauntsaninja merged commit b95b1b3 into python:main Dec 28, 2022

bedevere-bot removed the awaiting merge label Dec 28, 2022

miss-islington assigned hauntsaninja Dec 28, 2022

bedevere-bot removed the needs backport to 3.10 only security fixes label Dec 28, 2022

hauntsaninja added needs backport to 3.11 only security fixes and removed needs backport to 3.11 only security fixes labels Dec 28, 2022

bedevere-bot removed the needs backport to 3.11 only security fixes label Dec 28, 2022

slateny deleted the s/55688 branch December 28, 2022 06:47

hauntsaninja mentioned this pull request Apr 23, 2023

Python3 erroneously calls r'\' a syntax error #103704

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gh-55688: Add note about ending backslashes for raw strings #94768

gh-55688: Add note about ending backslashes for raw strings #94768

slateny commented Jul 12, 2022 •

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bedevere-bot commented Dec 28, 2022

	Note that escaping quotes in raw strings will keep the backslash::
	Note that, unlike the non–raw string case, escaping quotes in raw strings will keep the backslash::

gh-55688: Add note about ending backslashes for raw strings #94768

gh-55688: Add note about ending backslashes for raw strings #94768

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