Description

pganssle

opened on Sep 25, 2024

Issue body actions

Bug report

Bug description:

If you over-specify a datetime strptime string, it seems that datetime.strptime when encountering a component that has been specified twice, will overwrite earlier values with more recent ones.

>>> datetime.strptime("200099", "%Y%y")
datetime.datetime(1999, 1, 1, 0, 0)

One approach would be to ban over-specified formats, but that would be a problem for someone who wants to, use a format like "%M %B" (e.g. something sortable by month number while also showing month name), so I think we don't want to do that.

Alternatively we could start raising an exception if you get different values from these two things, or possibly a warning.

Or we can just accept that if you use a weird format you should expect weird behavior.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-124549: Document that strptime() uses the last directive when a time unit is over-specified #125009
• gh-124549: Warn on conflicting directives in datetime.*.strptime #132524
• [3.15] gh-124549: Document that strptime() uses the last directive when a time unit is over-specified (GH-125009) #159950
• [3.14] gh-124549: Document that strptime() uses the last directive when a time unit is over-specified (GH-125009) #159951

Activity

1. pganssle

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Sep 25, 2024
2. pganssle

   added this to Date and time issues 🕰️on Sep 25, 2024
3. Mariatta commented on Sep 26, 2024

   

   Mariatta

   on Sep 26, 2024

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   I'd be ok with raising exception, seems like it's a user error for passing these kind of conflicting format specifiers.

   We could also start documenting this case, saying that the latest format specifiers that appears will prevail.
4. Mariatta commented on Oct 5, 2024

   

   Mariatta

   on Oct 5, 2024

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   @pganssle I created a PR to document this weird behavior. I think it's still worth raising a warning though. #125009
5. alliesw commented on Dec 17, 2024

   

   alliesw

   on Dec 17, 2024

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   To avoid the issue where the last directive overwrites earlier values:
   since more than one directive is needed, maybe split the string and parse the components manually (handling the split and parsing based on the length of the string)?
   Maybe also apply custom logic to handle dynamic formats, especially if you encounter different scenarios with varying date formats.... (idk if your format is dynamic?)
6. alliesw commented on Dec 17, 2024

   

   alliesw

   on Dec 17, 2024

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   This is an example of what I'm getting at:

   def parse_date(input_str):
       if len(input_str) == 6:  # 4-digit year followed by 2-digit year
           full_year = input_str[:4]
           short_year = input_str[4:]
           year_full = datetime.strptime(full_year, "%Y")
           year_short = datetime.strptime(short_year, "%y")
           return year_short.replace(year=year_full.year)  # Use full year from the first part
       elif len(input_str) == 4:  # Only 4-digit year
           return datetime.strptime(input_str, "%Y")
       else:
           raise ValueError("Invalid date format")

   Example usage:

   input_str = "200099"
   parsed_date = parse_date(input_str)
   print(parsed_date)  # datetime.datetime(1999, 1, 1, 0, 0)
7. picnixz

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   on Dec 17, 2024
8. StanFromIreland commented on Apr 14, 2025

   

   StanFromIreland

   on Apr 14, 2025

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   This is a stdlib issue (strptime.py) the extension-modules label can be removed.
9. StanFromIreland

   removed

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   on Aug 29, 2025
10. Mariatta

    added 3 commits that reference this issue on Oct 20, 2026

    gh-124549: Document that `strptime()` uses the last directive when a …

    fd6ed15

    [3.15] gh-124549: Document that `strptime()` uses the last directive …

    8e00228

    [3.14] gh-124549: Document that `strptime()` uses the last directive …

    d621204
11. StanFromIreland

    closed this as completedon Oct 23, 2026
12. github-project-automation

    moved this to Done in Date and time issues 🕰️on Oct 23, 2026

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