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Recipe 3.7. Fuzzy Parsing of Dates
Credit: Andrea Cavalcanti
Problem
Your program needs to read and accept
dates that don't conform to the
datetime standard format of "yyyy, mm,
dd".
Solution
The
third-party dateutil.parser module provides a
simple answer:
import datetime
import dateutil.parser
def tryparse(date):
# dateutil.parser needs a string argument: let's make one from our
# `date' argument, according to a few reasonable conventions...:
kwargs = { } # assume no named-args
if isinstance(date, (tuple, list)):
date = ' '.join([str(x) for x in date]) # join up sequences
elif isinstance(date, int):
date = str(date) # stringify integers
elif isinstance(date, dict):
kwargs = date # accept named-args dicts
date = kwargs.pop('date') # with a 'date' str
try:
try:
parsedate = dateutil.parser.parse(date, **kwargs)
print 'Sharp %r -> %s' % (date, parsedate)
except ValueError:
parsedate = dateutil.parser.parse(date, fuzzy=True, **kwargs)
print 'Fuzzy %r -> %s' % (date, parsedate)
except Exception, err:
print 'Try as I may, I cannot parse %r (%s)' % (date, err)
if _ _name_ _ == "_ _main_ _":
tests = (
"January 3, 2003", # a string
(5, "Oct", 55), # a tuple
"Thursday, November 18", # longer string without year
"7/24/04", # a string with slashes
"24-7-2004", # European-format string
{'date':"5-10-1955", "dayfirst":True}, # a dict including the kwarg
"5-10-1955", # dayfirst, no kwarg
19950317, # not a string
"11AM on the 11th day of 11th month, in the year of our Lord 1945",
)
for test in tests: # testing date formats
tryparse(test) # try to parse
Discussion
dateutil.parser's
parse function works on a variety of date formats.
This recipe demonstrates a few of them. The parser can handle
English-language month-names and two- or four-digit years (with some
constraints). When you call parse without named
arguments, its default is to first try parsing the string argument in
the following order: mm-dd-yy. If that does not make logical sense,
as, for example, it doesn't for the
'24-7-2004' string in the recipe,
parse then tries dd-mm-yy. Lastly, it tries
yy-mm-dd. If a "keyword" such as
dayfirst or yearfirst is passed
(as we do in one test), parse attempts to parse
based on that keyword.The recipe tests define a few edge cases that a
date parser might encounter, such as trying to pass the date as a
tuple, an integer (ISO-formatted without spaces), and even a phrase.
To allow testing of the keyword arguments, the
tryparse function in the recipe also accepts a
dictionary argument, expecting, in this case, to find in it the value
of the string to be parsed in correspondence to key
'date', and passing the rest on to
dateutil's parser as keyword
arguments.dateutil's parser can provide a
pretty good level of "fuzzy"
parsing, given some hints to let it know which
piece is, for example, the hour (such as the AM in the test phrase in
this recipe). For production code, you should avoid relying on fuzzy
parsing, and either do some kind of preprocessing, or at least
provide some kind of mechanism for checking the accuracy of the
parsed date.
See Also
For more on date-parsing algorithms, see dateutil
documentation at ;
for date handling, see the datetime documentation
in the Library Reference.
