output filename formats don't work as expected

[Eli-the-Bearded]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.09.20
• I've searched the bugtracker for similar feature requests including closed ones

Description

Quoth the youtube-dl docs:

The basic usage is not to set any template arguments when downloading a single file, like in youtube-dl -o funny_video.flv "https://some/video". However, it may contain special sequences that will be replaced when downloading each video. The special sequences may be formatted according to python string formatting operations. For example, %(NAME)s or %(NAME)05d. To clarify, that is a percent symbol followed by a name in parentheses, followed by formatting operations.

Quoth the linked python string formatting operations:

's' String (converts any Python object using str()). (6)

6 If the object or format provided is a unicode string, the resulting string will also be unicode. The precision determines the maximal number of characters used.

$ youtube-dl -U
youtube-dl is up-to-date (2020.09.20)
$ youtube-dl -o '%(title)40s-%(id)s.%(ext)s' https://twitter.com/RebeccaRHelm/status/1318207763164573697
[twitter] 1318207763164573697: Downloading guest token
[twitter] 1318207763164573697: Downloading JSON metadata
[twitter] 1318207763164573697: Downloading m3u8 information
ERROR: unable to open for writing: [Errno 36] File name too long: "Open Ocean Exploration - In 2018 the @oceanexplorer saw THIS in the Gulf of Mexico!! According to their cephalopod biologist, it's \xe2\x80\x9cprobably the most bizarre squid I\xe2\x80\x99ve ever seen.\xe2\x80\x9d As far as I can tell we don't know much more about it! A stunning deep-sea mystery! \xf0\x9f\x8e\xa5https -_t.co_9vS5S0HLqC #DailyJelly-1318207763164573697.mp4.part"

If "maximal number of characters" can be used on strings, please document how it works since Python hasn't bothered. (I'm a C programmer, not a Python programmer.)

[dstftw]

It works exactly the same as C's printf, so that there must be a dot before precision.

[Eli-the-Bearded]

Did you fix the docs? No. I don't believe this should be closed. Here:

diff --git a/README.md b/README.md
index cd8856828..e4be8a972 100644
--- a/README.md
+++ b/README.md
@@ -594,7 +594,9 @@ To use percent literals in an output template use `%%`. To output to stdout use
 
 The current default template is `%(title)s-%(id)s.%(ext)s`.
 
-In some cases, you don't want special characters such as 中, spaces, or &, such as when transferring the downloaded filename to a Windows system or the filename through an 8bit-unsafe channel. In these cases, add the `--restrict-filenames` flag to get a shorter title:
+In some cases, you don't want special characters such as 中, spaces, or &, such as when transferring the downloaded filename to a Windows system or the filename through an 8bit-unsafe channel. In these cases, add the `--restrict-filenames` flag to get a shorter title.
+
+In some other cases the title may make the file name too long for the system. Specifying a maximum length for the title is one option, eg limit the title part to 60 characters: `%(title).60s-%(id)s.%(ext)s`.
 
 #### Output template and Windows batch files

[Eli-the-Bearded]

That was my request after all: please document how it works