The docs says

# %b or %B or %h
The month name according to the current locale, in abbreviated form or the full name.

In context of your question I parse that as:

%b  => month name in current locale
%B  => abbreviated form
%h  => full name (whatever that may mean)
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Update: And it turns out that I'm wrong. Changing %b to %B doesn't work. The test suite uses this snippet:

21:     ['%b %d, %Y', 'Jan 24, 2003'],
22:     ['%B %d, %Y', 'January 24, 2003'],
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So it seems that %b is really meant to be the short form, and %B is meant to be long form.

Separating the month from the year with a blank in both the format and the input string still gives an error message though:

There is no use providing a month name (Jul) without providing a year
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So maybe the problem is actually with the year?