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MrHaxtar opened this issue Nov 7, 2022 · 4 comments
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Documentation for str.count() should mention the empty string case #99183

MrHaxtar opened this issue Nov 7, 2022 · 4 comments
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docs Documentation in the Doc dir

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@MrHaxtar
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MrHaxtar commented Nov 7, 2022 •
edited by bedevere-bot
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This is the bug in python count() function.

>>> a="I Iove python"
>>> b=a.count("")
>>> print(b)

So normally count function is used to check specific word into the string. So if I am using "" it will giving output as 14 instead of showing error. Anyone knows how to handle this bug or may be it's not fixed till now by www.python.org
@MrHaxtar MrHaxtar added the type-bug An unexpected behavior, bug, or error label Nov 7, 2022
@hauntsaninja
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This is intentional, you can see that this case is explicitly handled here:

cpython/Objects/stringlib/count.h

Line 22 in df3a6d9

return (str_len < maxcount) ? str_len + 1 : maxcount;

And this preserves behaviour going back at least 22 years, e.g. see:

cpython/Objects/unicodeobject.c

Line 1864 in d57fd91

int count(PyUnicodeObject *self,

In case it helps explain the behaviour, "" is a substring of all strings and e.g. if it's true that "" in string, then it makes sense that string.count("") > 0. While the code today is a little complicated, if you look at my second link from the code 23 years ago, it's pretty easy to understand what count is doing and why it gets 14.

@stevendaprano
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This is the bug in python count() function.

It is not a bug. The empty string matches 14 positions of your string:

  1. An empty string matches before the "I"
  2. And after the "I" and before the first space.
  3. And after the space and before "l".
  4. And between the "l" and the "o".

and so on. If you count them, there are 14 positions where an empty string matches.

See also this Stackoverflow answer.

@stevendaprano stevendaprano changed the title #bug_report:Failed to give error count("") Documentation for str.count() should mention the empty string case Nov 7, 2022
@stevendaprano stevendaprano added docs Documentation in the Doc dir and removed type-bug An unexpected behavior, bug, or error labels Nov 7, 2022
@stevendaprano
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This issue comes up fairly regularly and many people seem to be surprised by it. I think it might help for the docs to explicitly mention that the empty string matches every position in the string, and so str.count("") returns one more than the length of the string.

@MrHaxtar
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MrHaxtar commented Nov 8, 2022

This is the bug in python count() function.

It is not a bug. The empty string matches 14 positions of your string:

  1. An empty string matches before the "I"
  2. And after the "I" and before the first space.
  3. And after the space and before "l".
  4. And between the "l" and the "o".

and so on. If you count them, there are 14 positions where an empty string matches.

See also this Stackoverflow answer.

Thanks for the information

ramvikrams added a commit to ramvikrams/cpython that referenced this issue Nov 9, 2022
@ramvikrams
pythongh-99183: Documentation for str.count() updated
7d2d7ce 
Updated the documentation for str.count() and added a example to explain the documentation better.
@ramvikrams ramvikrams mentioned this issue Nov 9, 2022
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rhettinger added a commit to rhettinger/cpython that referenced this issue Nov 10, 2022
@rhettinger
pythonGH-99183: Document behavior of count() for empty substrings
5387fdc
@bedevere-bot bedevere-bot mentioned this issue Nov 10, 2022
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rhettinger added a commit that referenced this issue Nov 10, 2022
@rhettinger
GH-99183: Document behavior of count() for empty substrings (GH-99339)
2f4af2d
miss-islington pushed a commit to miss-islington/cpython that referenced this issue Nov 10, 2022
@rhettinger @miss-islington
pythonGH-99183: Document behavior of count() for empty substrings (py…
78c354f 
…thonGH-99339)

(cherry picked from commit 2f4af2d)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
@bedevere-bot bedevere-bot mentioned this issue Nov 10, 2022
Merged
miss-islington pushed a commit to miss-islington/cpython that referenced this issue Nov 10, 2022
@rhettinger @miss-islington
pythonGH-99183: Document behavior of count() for empty substrings (py…
f809146 
…thonGH-99339)

(cherry picked from commit 2f4af2d)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
@bedevere-bot bedevere-bot mentioned this issue Nov 10, 2022
Merged
miss-islington added a commit that referenced this issue Nov 10, 2022
@miss-islington @rhettinger
GH-99183: Document behavior of count() for empty substrings (GH-99339)
f7d3b18 
(cherry picked from commit 2f4af2d)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
miss-islington added a commit that referenced this issue Nov 10, 2022
@miss-islington @rhettinger
GH-99183: Document behavior of count() for empty substrings (GH-99339)
17f1c0a 
(cherry picked from commit 2f4af2d)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
ethanfurman pushed a commit to ethanfurman/cpython that referenced this issue Nov 12, 2022
@rhettinger @ethanfurman
pythonGH-99183: Document behavior of count() for empty substrings (py…
660e603 
…thonGH-99339)
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