Understanding and Avoiding Common Mistakes with Python datetime.strptime and strftime

Even experienced Python users frequently reverse the arguments of datetime.strptime(string, format) , writing datetime.strptime(format, string) , and confuse it with re.find(pattern, string) .

Analysis of the problem

1. datetime.strptime()

The function name is ambiguous; a clearer name like str2time would emphasize that a source string is parsed into a time object. The official name includes the verb parse , highlighting its core logic.

<code>strptime(string, format) method of builtins.type instance
    string, format -> new datetime parsed from a string (like time.strptime())
#Case
In [6]: datetime.strptime("8 May, 2019", "%d %B, %Y")
Out[6]: datetime.datetime(2019, 5, 8, 0, 0)
</code>

The function works by parsing a string to produce a time object, following the source‑to‑destination pattern.

We can extend the parameter names for clarity:

<code>datetime.strptime(string, format)
# extended as
def strptime(src_string, parsed_format) -> "dst_time_object":
    ...
</code>

Because re.match(pattern, string) places the pattern first, some developers mistakenly put the format first in strptime . However, strptime(string, format) adheres to the conventional source‑to‑destination order.

2. datetime.strftime()

The inverse function strftime(format) (or time2str ) also follows the source‑to‑destination principle, which can lead to the same confusion when users write the format before the string.

<code>In [8]: dt
Out[8]: datetime.datetime(2019, 5, 8, 0, 0)
In [9]: dt.strftime("%d %B, %Y")
Out[9]: '08 May, 2019'
</code>

Using the functional form:

<code>In [9]: datetime.strftime(datetime.now(), "%b %d %Y %H:%M:%S")
Out[9]: 'Jun 11 2020 08:08:52'
</code>

Both functions thus respect the src to dst rule.

3. Special cases and regular patterns

Besides regex methods, most string‑related functions follow the source‑to‑destination pattern, e.g., str.split() and str.join() . The latter is a special case where the output (the joined string) appears before the input list.

<code>In [12]: "source to destination".split()
Out[12]: ['source', 'to', 'destination']  # src to dst
In [13]: " ".join(['source', 'to', 'destination'])
Out[13]: 'source to destination'
</code>

Python provides many more string methods than list methods, reflecting its focus on string processing.

<code>In [16]: len([m for m in dir(str) if not m.startswith("__")])
Out[16]: 45
In [17]: len([m for m in dir(list) if not m.startswith("__")])
Out[17]: 11
</code>

4. System date and clock services

Maintaining correct system time is a core OS service, and the same parsing/formatting logic applies to command‑line tools:

<code>$ date -u
$ sudo hwclock -u
2019-05-08 20:19:14.764965+08:00  # str2time, strptime
$ date -d "08 May 2019" +"%c"
Wed 08 May 2019 14:00:00 AM CST  # time2str, strftime
$ date +"%d %B, %Y"
08 May, 2019
</code>

Hope this analysis helps you avoid common pitfalls when using strptime and strftime in Python.