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simple-date

A wrapper around the datetime, pytz and tzlocal packages for Python 3.2+. Full docs below.

Examples

Just give me a UTC datetime for these dates!

>>> for date in '1/6/2013 BST', '1/6/2013 EST', 'Tue, 18 Jun 2013 12alwa:19:09 -0400':
>>>     print(best_guess_utc(date))
2013-05-31 23:00:00+00:00
2013-01-06 05:00:00+00:00
2013-06-18 16:19:09+00:00

What time is it now, in New York?

>>> SimpleDate(tz='America/New_York')
SimpleDate('2013-06-14 13:14:17.295943 EDT', tz='America/New_York')

And what time is that in the UK (the country code is for Great Britain)?

>>> SimpleDate('2013-06-14 13:14:17.295943 EDT').convert(country='GB')
SimpleDate('2013-06-14 18:14:17.295943 BST', tz='Europe/London')

What is the UTC for this email date?

>>> SimpleDate('Fri, 14 Jun 2013 13:13:42 -0400').utc
SimpleDate('Fri, 14 Jun 2013 17:13:42 +0000', tz='UTC')

What's the date a week from now (I live in Chile)?

>>> SimpleDate() + timedelta(days=7)
SimpleDate('2013-06-21 13:55:20.791519 CLT', tz='America/Santiago')

The day of the week for Xmas this year?

>>> SimpleDate(2013, 12, 24).weekday
1

And as a naive datetime?

>>> SimpleDate(2013, 12, 24).naive.datetime
datetime.datetime(2013, 12, 24, 0, 0)

What's the time in EST for epoch 1234567890?

>>> SimpleDate(1234567890, tz='EST')
AmbiguousTimezone: 3 distinct timezones found: <'EST'>; <'Australia/NSW'>; ...

Whoa! What are those crazy Australians doing? Let's force the USA (only):

>>> SimpleDate(1234567890, tz='EST', country='US', format=MDY)
SimpleDate('02/13/2009 18:31:30.000000 EST')

Alternatively, we could give priority to the USA and take the first solution we find:

>>> SimpleDate(1234567890, tz='EST', country=prefer('US'), unsafe=True, format=MDY)
SimpleDate('02/13/2009 18:31:30.000000 EST')

And what day is that?

>>> SimpleDate(1234567890, tz='EST', country=prefer('US'), unsafe=True).strftime('%A')
'Friday'

Documentation

Installation

Please install from PyPI.

First, optionally, make a virtualenv:

virtualenv env
source ./env/bin/activate

Then install all the packages:

pip install pytz tzlocal simple-date

Common Concepts and Parameters

Timezone - tz

You can specify a timezone in various different ways (I've omitted any date in the examples below, so the results show "now" when I was writing these docs):

  • a name, as a string;

    >>> SimpleDate(tz='EDT')
    SimpleDate('2013-06-17 19:48:35.556400 EDT')
    >>> SimpleDate(tz='America/New_York')
    SimpleDate('2013-06-17 19:48:43.662401 EDT', tz='America/New_York')
  • an offset relative to UTC in minutes or as a timedelta instance (I'm specifying the format to switch from name, %Z, to numerical offset, %z, as these tzinfo instances don't have a name);

      >>> SimpleDate(tz=120, format='%Y-%m-%d %z')
      SimpleDate('2013-06-18 +0200', tz='pytz.FixedOffset(120)')
      >>> SimpleDate(tz=datetime.timedelta(minutes=60), format='%Y-%m-%d %z')
      SimpleDate('2013-06-18 +0100', tz='pytz.FixedOffset(60)')
  • using an existing tzinfo instance;

    >>> SimpleDate(tz=pytz.timezone('UTC'))
    SimpleDate('2013-06-17 23:50:59.141497 UTC', tz='UTC')
  • and if you give a tuple, any value might be used.

    >>> SimpleDate(tz=('CLT', 'CLST'))
    SimpleDate('2013-06-17 19:52:17.384333 CLT')

Finally, if you give a timezone that conflicts with the timezone in another parameter, you're going to have a bad time:

>>> SimpleDate('2013-06-17 EDT', tz='America/New_York')
SimpleDate('2013-06-17 EDT', tz='America/New_York')
>>> SimpleDate('2013-06-17 EDT', tz='America/Santiago')
simpledate.NoTimezone: No timezone found (timezones=['EDT', 'America/Santiago'], ...)

Daylight Saving Time - is_dst

Is the date currently being processed in daylight savings?

This may seem an odd thing to specify, but is needed when the clocks "go back" in a geographic timezone. For example, in Chile, daylight saving time 2012 ended at midnight on Sunday, 28 April. At that moment the clocks went back an hour and it was 11pm again! So the time 11:30pm on Sunday 8 September in the timezone America/Santiago was ambiguous. And the parameter is_dst will resolve that ambiguity if you happen to need it:

>>> fmt = '%H:%M %Z'
>>> SimpleDate('2012-04-28 23:30', tz='America/Santiago', is_dst=False, format=fmt)
SimpleDate('23:30 CLT', tz='America/Santiago')
>>> SimpleDate('2012-04-28 23:30', tz='America/Santiago', is_dst=True, format=fmt)
SimpleDate('23:30 CLST', tz='America/Santiago')

If you set this to None, then, when needed, you get an error:

>>> SimpleDate('2012-04-28 23:30', tz='America/Santiago', is_dst=None)
pytz.exceptions.AmbiguousTimeError: 2012-04-28 23:30:00

So by default it is set to False, which avoids unexpected errors at the cost of resolving them to normal (ie not daylight saving) time.

Country Code - country

Giving a country code, or tuple of codes, restricts the search for a timezone to the timezones used in those countries.

For example, the timezone EST means something different in the USA and Australia:

>>> SimpleDate('2013-01-01', tz='EST')
AmbiguousTimezone: 3 distinct timezones found: <'EST'>; <'Australia/Melbourne'>; ...

We can remove this ambiguity by specifying the country code 'US':

>>> SimpleDate('2013-01-01', tz='EST', country='US')
SimpleDate('2013-01-01')

Two useful helper functions are prefer(code, code, ...) and except(code, code, ...) whose results can be passed to country=.... The prefer(...) function returns all country codes, but places the given codes first, while except(...) returns all codes except those given as arguments.

TZ Factory - tz_factory

Provide a PyTzFactory that is used to find the timezone. Otherwise, by default, all calls to the API use the DEFAULT_TZ_FACTORY instance.

This is useful in two cases:

  1. With multiple threads. The code is not thread safe, so if you are creating SimpleDate instances in multiple threads then each thread must have its own factory.

    >>> local_factory = PyTzFactory()
    >>> SimpleDate('2013-06-18', tz_factory=local_factory)
  2. To give exact control over which timezones are used. For example, to use only timezones with an 'x':

    >>> x_timezones = PyTzFactory([z for z in pytz.all_timezones if 'x' in str(z)])
    >>> SimpleDate('2013-06-18 XYZ', tz_factory=x_timezones)

First Found - unsafe

Setting unsafe=True will return the first timezone found. This is dangerous because no exception is generated for ambiguous values, but it also has two advantages:

  1. It's faster. The timezone factory doesn't search all possibilities.

  2. Because country sets the order in which the zones are checked, this gives control over how ambiguous timezones are resolved.

    A good example is the timezone EST, which can be used in both the USA and Australia. If we set country=prefer('US') then the American timezones are checked first:

    >>> SimpleDate(1234567890, tz='EST')
    AmbiguousTimezone: 3 distinct timezones found: <'EST'>; <'Australia/NSW'>; ...
    >>> SimpleDate(1234567890, tz='EST', country=prefer('US'), unsafe=True)
    SimpleDate('2009-02-13 18:31:30.000000 EST')

Format - format

The format used to parse and display strings. For display, this is the same as the standard Python format.

For parsing, it has been extended to:

  • Grouping and alternatives with %(...%|...%|...%). For example '%(%Z%|%z%)' would match either timezone name or a numeric offset.

  • Optional values with %?. For example '%H %?%M' is hours and minutes with an optional space between.

  • Directives for symbols and text can be modified to be more lenient by adding a ! after the %. For example '%!Z' will match a wide variety of timezone names (the default is to match only those know to the current locale), and '%!a' will match any word (not just days of the week in the current locale).

  • Directives for digits can be modified to be more strict by adding a ! after the %. For example, '%!d' will only match two digits (while '%d' will also match a single digit, possibly with a leading space).

  • Since the changes above result in an unreadable soup of % and ! signs, the invert(...) function will add % to templates where missing, and remove where present. Sounds crazy, but simplifies things hugely. Here's an ISO8601-like format with a letter "T" in the middle and optional seconds:

    >>> SimpleDate('2013-06-24T17:47', format=invert('Y-m-d%TH:M(:S)?'), tz=utc)
    SimpleDate('2013-06-24T17:47', tz='UTC')

    (incidentally, that format inverts to '%Y-%m-%dT%H:%M%(:%S%)%?').

  • To make life even simpler, when the format contains no % signs then invert(...) is automatically applied:

    >>> SimpleDate('2013-06-27 17:47', format='Y-m-d H:M(:S)?', tz=utc)
    SimpleDate('2013-06-24 17:47', tz='UTC')

When passed to the SimpleDate constructor, the format is used both to parse dates and to format them:

   >>> birthday = SimpleDate('19 May 2013', format='d b Y')
   >>> str(birthday)
   19 May 2013

When an extended format is used for parsing, Simple Date uses the matched data to select a format for printing. So if '%(%H:%)?%M' matched both hours and minutes then the format would be '%H:%M', but if it matched only minutes then the format for printing would be '%M'.

Whether a format is supplied or not, the formats in the relevant SimpleDateParser (by default, DEFAULT_DATE_PARSER) can also be used to parse the string, if necessary. And if no format is supplied then the format in the parser that succeeds is used for formatting output too.

Multiple formats can be given as a tuple. If more than one format is given then the format that successfully parses the value will be used for display (unlike the single value case, which is always used for display, even if another format from date_parser is used for parsing).

   >>> SimpleDate('6/12/2013', format=DMY + YMD)
   SimpleDate('06/12/2013', tz='America/Santiago')

Date Parser - date_parser

Give the SimpleDateParser that will be used when parsing a string. This lets you specify which date formats are used in parsing - important because American and European date styles conflict.

For example, to parse American style (month/day/year):

>>> american = SimpleDateParser(MDY + DEFAULT_FORMATS)
>>> SimpleDate('12/24/2013', date_parser=american)
SimpleDate('12/24/2013', tz='America/Santiago')

Debugging - debug

Setting debug=True will display a lot of information on stdout.

For example, we can see why the following fails:

>>> SimpleDate('2013-01-01 CLT', country='CL', debug=True)
SimpleDate: Inferring auto argument
SimpleDate: Found a string, will try to parse
SimpleDate: Using default date parser
SimpleDateParser: Raw parse results for %Y-%m-%d %Z: (2013, 1, 1, 0, 0, 0, 1, 1, 0, 'CLT', None), 0
SimpleDateParser: Parsed timezone name from date as CLT
SimpleDateParser: Combined zones are ('CLT',)
PyTzFactory: Searching (timezones=('CLT',), datetime=datetime.datetime(2013, 1, 1, 0, 0), is_dst=False, country='CL', unsafe=False)
PyTzFactory: Have country code CL
PyTzFactory: Country code CL has 2 timezones
PyTzFactory: Expanded country codes to 2 timezones
PyTzFactory: Expanding ('CLT',)
PyTzFactory: Name lookup failed for CLT
PyTzFactory: America/Santiago gave CLST
PyTzFactory: Pacific/Easter gave EASST
PyTzFactory: Expanded timezone to 0 timezones
[...traceback...]
simpledate.NoTimezone: No timezone found (timezones=('CLT',), datetime=datetime.datetime(2013, 1, 1, 0, 0), is_dst=False, country='CL', unsafe=False)

The format is handled correctly, but the expected timezone, America/Santiago, is giving CLST instead of CLT - Chile is in the Southern Hemisphere so it's summer in January.

Complete API

SimpleDate

SimpleDate is an immutable wrapper around a datetime, tzinfo, and format string. The constructor can be used to convert from other formats; instance attributes can be used to convert to those formats.

Here is an example that combines a date and time in the EDT timezone and then converts the result to a datetime in UTC:

>>> SimpleDate(date=date(2013, 6, 15), time=time(10, 50), tz='EDT').utc.datetime
datetime.datetime(2013, 6, 15, 14, 50, tzinfo=<UTC>)

Constructor

For something that calls itself "simple", the SimpleDate constructor is a monster. However, it's a lot easier to understand when you group the parameters by functionality: there are the numerical values used to specify a date and time; the conversions that construct a SimpleDate from some other type; and then the standard arguments (tz, is_dst, country, tz_factory, unsafe, format, date_parser, and debug) that were described earlier.

Numerical Values

These are year, month, day, hour, minute, second and microsecond and they are in that order at the start of the constructor, so you don't need to name them. For example:

>>> SimpleDate(2013, 6, 12, 0, 37, tz='EDT')
SimpleDate('2013-06-12 00:37:00.000000 EDT')

You must supply at least year, month and day. Missing values (on the right) default to zero.

Conversions

Alternatively, instead of using numerical values, you can supply one of these parameters:

  • simple - Another SimpleDate instance (which will be copied)

  • datetime - A datetime instance from the standard datetime package.

  • date - A date instance from the standard datetime package.

  • ordinal - A Gregorian ordinal (it's used in the standard datetime package).

  • time - A time instance from the standard datetime package.

  • timestamp - A Posix timestamp (Unix epoch).

In general, only one of these can be provided. The exception is that time can be combined with date or ordinal.

If only a single value is given then (except for ordinal) it can be given as the first argument in the constructor - the type of the value will be used to work out how it should be handled (the code knows it's not year because there's no month or day).

For example:

>>> SimpleDate(datetime(2013, 6, 12, 0, 37), tz='EDT')
SimpleDate('2013-06-12 00:37:00.000000 EDT')

Attributes

Attributes can be described as simple or complex:

Simple Attributes

These are all pretty obvious - the names usually match the standard Python APIs, but everything is an attribute.

  • datetime, date, time, ordinal, timestamp

  • year, month, day, hour, minute, second, millisecond

  • weekday, isoweekday, isocalendar

  • tzinfo

  • format

Complex Attributes

They return "higher level" objects with modified timzone information.

  • naive - A wrapper around a naive datetime (one without tzinfo set). The wrapper contains all the attributes described above. So, for example, mysimpledate.naive.datetime gives the naive datetime.

  • utc - An equivalent SimpleDate in the UTC timezone. Because this is another SimpleDate instance it also contains all the attributes described here.

  • normalized - An equivalent SimpleDate, in UTC, with a standard format (so similar to .utc, but with the format changed too). Useful for comparisons (see below).

Operators

SimpleDate supports similar operations to datetime: addition with timedelta; subtraction of timedelta or other SimpleDate instances; comparison; equality.

IMPORTANT Equality includes the timezone and format. So for consistent comparison, convert to UTC with a standard format first. The normalized attribute does this (see above).

Conversion

For conversion to a string, SimpleDate supports .strftime(format) which takes a standard format string:

>>> SimpleDate(2013, 12, 24).strftime('%A')
'Tuesday'

For conversion to other dates, SimpleDate has a method, .convert(...), which takes the usual parameters (tz, is_dst, country, tz_factory, unsafe, format, debug) and returns a new SimpleDate with the same equivalent time, but matching the new requirements.

So, for example, to change format:

>>> default_fmt = SimpleDate(datetime(2013, 6, 17))
>>> str(default_fmt)
'2013-06-17 00:00:00.000000 CLT'
>>> short_fmt = default_fmt.convert(format='Y-m-d')
>>> str(short_fmt)
'2013-06-17'

In the example above, we can use Y-m-d rather than %Y-%m-%d thanks to the implicit inversion of formats.

For conversion to other types, see attributes.

Replacement

Similar to datetime.replace(), SimpleDate has a .replace(...) method that generates a new time (unlike .convert(...), which is the same time in a different timezone or format).

This combines the usual datetime arguments (year, month, day, hour, minute, second, microsecond) with the standard SimpleDate parameters that control timezone resolution etc (tz, is_dst, country, tz_factory, unsafe, format, debug).

For example, to move to the start of the day:

>>> SimpleDate().replace(hour=0, minute=0, second=0, microsecond=0)
SimpleDate('2013-06-22 00:00:00.000000 CLT', tz='America/Santiago')

SimpleDateParser

Often you need to parse dates without knowing, ahead of time, the exact date format. One approach is to try write code that is "smart enough" to parse many formats. That approach is taken by python-dateutil (I believe). An alternative, which may be slower, but also more reliable, is to try different formats in turn. This latter approach is taken by SimpleDateParser.

Note that the SimpleDate constructor calls SimpleDateParser automatically (either DEFAULT_DATE_PARSER, or the instance supplied as date_parser=...). You only need to use this class directly if you want to use a different set of formats from the default.

Constructor

The constructor takes a list of formats, which will be tried until one works (the order is not guaranteed - more successful formats are tried first).

Predefined lists include RFC_2822 (aliased as EMAIL), ISO_8601 (aliased as YMD), ASN_1, MDY and DMY.

MDY and DMY are mutually exclusive - only use one at a time.

The default is DEFAULT_FORMATS = ISO_8601 + RFC_2822 + ASN_1

Note that SimpleDateParser is not a validating parser. By design, these formats will match many dates that are inconsistent with the associated specifications. If you want to check for an exact match with a format, create a SimpleDateParser with a single, explicit format. And even then, a space in the format may match multiple spaces in the input string (this is also true when using the standard Python parser).

Parsing

SimpleDateParser has a single method, .parse(...) which takes a date (as a string) plus the usual suspects (tz, is_dst, country, tz_factory, unsafe, format, debug) and returns a SimpleDate instance.

PyTzFactory

The PyTzFactory is responsible for finding a timezone that matches various constraints - things like the timezone name, the date in question, and, perhaps, a set of countries.

Sometimes this is easy: if it's given a tzinfo instance and no other constraints it simply returns the value. But in general it requires a search through all the available timezones.

As with SimpleDateParser, this is called from the SimpleDate constructor via DEFAULT_TZ_FACTORY (or given by tz_factory=...).

Constructor

The constructor for PyTzFactory takes a list of timezones (by default pytz.common_timezones) and countries (by default, None, implying all). From this it constructs a set of common timezones that will be used to search for values.

Timezone Search

The .search(...) method takes zero or more timezones (unnamed arguments), a datetime instance (datetime=... - defining when the timezone is used), and the standard is_dst, country, unsafe, and debug. It searches for, and returns, a timezone (tzinfo instance) that matches the parameters given.

The timezone arguments are typically strings, although integer minutes, tzinfo and timedelta instances are also supported. Also, several can be grouped in a tuple (see below).

Each timezone is used in sequence to restrict the range of possible values (functions as a logical AND). A tuple timezone matches any of the values (functions as a logical OR). So, for example, if called as

>>> PyTzFactory().search(A, (B, C))

then the result will be consistent with A and (B or C).

Other Methods

These are mainly for internal use:

  • .distinct(...) - Filter timezones, returning those with distinct offsets from UTC at the time given.

  • .expand_tz(...) - Implement search for a single timezone (or tuple).

  • .expand_country(...) - Expand country codes to their associated timezones.

Functions for tzinfo

The following functions are more robust (or perhaps I misunderstood the API) replacements for various tzinfo methods:

  • reapply_tzinfo(datetime, is_dst) - A more powerful tzinfo.normalize(...).

  • tzinfo_astimezone(tzinfo, datetime) - Sets tzinfo after conversion.

  • tzinfo_tzname(tzinfo, datetime, is_dst) - Handles optional is_dst and returns values for fixed offsets.

  • tzinfo_utcoffset(tzinfo, datetime) - Handles optional is_dst.

  • tzinfo_localize(tzinfo, datetime, is_dst) - Handles optional is_dst.

Best Guess UTC

best_guess_utc(date, debug=False) is a helper function for the most common use-case: given some input (in any of the formats supported by the SimpleDate constructor), return the most likely datetime in UTC. It is a wrapper around the other classes here which attempts to parse American-style (month first) dates in US timezones (only). If that fails then it uses other timezones with European-style (day first) dates.

The implementation uses unsafe=True (docs) and thread-local factories (so can be called from multiple threads). It is intended to be efficient and robust, but may sacrifice accuracy in ambiguous cases.

FAQ

What is the Licence?

Simple Date is (c) 2013 Andrew Cooke (andrew@acooke.org). It is released into the public domain for any use, but with absolutely no warranty.

Are You Supporting this Code?

Yes (contact at me at andrew@acooke.org if you have a bug report). BUT this is something of an experiment. The API of future versions could change significantly (earlier alpha versions changed a lot as I understood more about timezones and the problems involved).

The challenge is to make something that is simple, general, and correct...

Why Python 3.2+?

The code uses OrderedDict (3.1+) and datetime.timezone (3.2+).

Is the Library Thread Safe?

NO. Both SimpleDateParser and PyTzFactory mutate their contents to improve efficiency on repeated calls. This will give undefined behaviour if they (ie DEFAULT_DATE_PARSER and DEFAULT_TZ_FACTORY) are called from multiple threads.

If you do use multiple threads, you must create instances of those classes for each thread and pass them to SimpleDate using date_parser=... and tz_factory=....

However, best_guess_utc is thread safe (the implementation uses the approach above via thread-local storage).

Why Did I Get the Error "Could not parse ..."?

SimpleDate does not know the format for the string you gave. Specify the format using format=... in the SimpleDate constructor.

By default neither American nor European formats are included (they conflict) so if you want to parse European style dates:

>>> SimpleDate('24/12/2013', format=DMY)
SimpleDate('24/12/2013', tz='America/Santiago')

Or, more if you want to be more efficient with multiple dates (and more flexible - we're including ISO 8601 and RFC 2822 defaults too):

>>> european = SimpleDateParser(DMY + DEFAULT_FORMATS)
>>> SimpleDate('24/12/2013', date_parser=european)
SimpleDate('24/12/2013', tz='America/Santiago')

Why Did I Get the Error "No timezone found"?

The timezone you gave was not found in the PyTZ database. This may be because it was given for a time that doesn't make sense (for example, using daylight savings in winter). Using debug=True can sometimes be useful in understanding what is going wrong.

Why Did I Get the Error "AmbiguousTimezone: ..."?

The timezone you gave could be matched against more that one timezone in PyTZ's database. See the need for search. Possible solutions include using the unsafe or country parameters.

Why Did I Get the Error "SingleInstantTzError ..."?

You tried to use a tzinfo instance that is defined only for one moment in time. About all you can do with such dates is convert them to UTC. See the need for search.

What is the Best Way to Use this Library?

Parse and convert to UTC.

Background

This section tries to explain and justify the library's implementation.

Classifying Timezones

I am no expert on timezones. Everything I know I have inferred from the PyTZ API. Here I am going to impose some structure on the different kinds of timezone. The aim is to construct a vocabulary within which the implementation can be explained.

Four aspects of timezones seem to be important. The first two are:

  • Temporal Timezones define fixed offsets from UTC. They can be written like +0300 or UTC+3 (3 hours ahead of UTC). Some are given names. For example, EDT (Eastern Daylight Time) is UTC-4.

  • Geographical Timezones are associated with a particular place. They are typically defined by some official body and may have a history. In particular, many geographical timezones make an adjustment (daylight saving time) during the summer, and the dates when that adjustment is made may change from year to year. So geographical timezones are often defined in terms of temporal timezones. Finally, geographical timezones can be written like America/New_York.

It follows from all this that (1) geographical timezones are defined throughout the year (but may be ambiguous - see is_dst), and (2) some temporal timezones may only be valid at certain times.

Here is an example of an invalid temporal timezone: "2013-06-01 CLST". June is the middle of winter in Chile (in the Southern Hemisphere) and so a June date would not be expressed in terms of Chilean Summer Time.

But not all temporal timezones behave this way. UTC is always valid, for example. So from this we have two more ways to classify timezones:

  • Unlimited Timezones are always valid. All geographical timezones, and some temporal timezones, are unlimited.

  • Limited Timezones are only valid at certain times. These are temporal timezones. Daylight saving times are limited, temporal timezones, for example.

Now, finally, we have the tools to understand PyTZ. The PyTZ package is a database of unlimited timezones. If we have the name of an unlimited timezone, we can extract a tzinfo instance (tzinfo is a standard Python class from the datetime package) that will give us the temporal timezone on any date.

That's great, and PyTZ is very useful. But...

...unfortunately, when dates are presented as text, they typically use limited, temporal timezones. These are not provided directly by PyTZ - they appear only when applying a geographical timezone to a particular date.

As an example. In PyTZ is easy, if we know someone is in America/New_York, and that their current the date/time is 2013-06-01 12:00, to infer that it should be written as EDT.

But if someone writes 2013-06-01 12:00 EDT, what is their geographical timezone? And what timezone would they use for 2013-01-01 12:00, in the middle of winter? These kinds of questions are harder to answer with PyTZ.

[Aside: This is not the "fault" of PyTZ, as far as I can tell. It's simply how the Olson timezone database works, which presumably that reflects the real-life complexities of timezones.]

The Need for Search

If we are parsing a limited, temporal timezones then we cannot retrieve it directly from PyTZ (see above). Instead, we need to go through the the database of unlimited timezones, trying each in turn, until we find an unlimited timezone which, at the particular date we are parsing, uses the limited timezone we have.

An example might clear things up. When SimpleDateParser reads a date string containing EDT then it calls PyTzFactory, which runs through the different timezones from PyTZ until it finds one - America/New_York, for example - that would be displayed as EDT on the date in question.

This process can have problems:

  1. No suitable timezone is found. In this case a NoTimezone exception is raised.

    >>> SimpleDate('2013-06-17 BAD')
    NoTimezone: No timezone found (timezones=('BAD',), datetime=datetime.datetime(2013, 6, 17, 0, 0), is_dst=False, country=None, unsafe=False)
  2. Multiple timezones are found, but they are all at the same offset from UTC. For example, in the case of EDT this might include America/New_York and America/Detroit (amongst others). In this case, a SingleInstantTz is created - a tzinfo instance that is valid only at the time we searched for.

    >>> SimpleDate('2013-06-17 EDT', debug=True)
    ...
    PyTzFactory: New offset -1 day, 20:00:00 for America/Indiana/Marengo
    PyTzFactory: Known offset -1 day, 20:00:00 for America/Thunder_Bay
    PyTzFactory: Known offset -1 day, 20:00:00 for America/Toronto
    PyTzFactory: Known offset -1 day, 20:00:00 for America/Indiana/Winamac
    ...
    SimpleDate('2013-06-17 EDT')
    >>> SimpleDate('2013-06-17 EDT').tzinfo
    SingleInstantTz(datetime.timedelta(-1, 72000), 'EDT', datetime.datetime(2013, 6, 17, 4, 0, tzinfo=<UTC>))
  3. Multiple timezones with different offsets from UTC are found. In this case an AmbiguousTimezone exception is raised (unless unsafe is set):

    >>> SimpleDate('2013-06-17 EST')
    AmbiguousTimezone: 2 distinct timezones found: <StaticTzInfo 'EST'>; <DstTzInfo 'Australia/Sydney' EST+10:00:00 STD> (timezones=('EST',), datetime=datetime.datetime(2013, 6, 17, 0, 0), is_dst=False, country=None, unsafe=False)

A SingleInstantTz is also returned on success when unsafe=True is used (which returns the first timezone found), because it is unclear whether the result is case 2 (or even 3, hence the name 'unsafe').

>>> SimpleDate('2013-06-17 America/New_York').tzinfo
<DstTzInfo 'America/New_York' EDT-1 day, 20:00:00 DST>
>>> SimpleDate('2013-06-17 America/New_York', unsafe=True).tzinfo
SingleInstantTz(datetime.timedelta(-1, 72000), 'EDT', datetime.datetime(2013, 6, 17, 4, 0, tzinfo=<UTC>))

When a SingleInstantTz is used for anything except working with the date that was used when it was created, a SingleInstantTzError is raised. This is because at other times case (2) above may change to case (3). We have no way of knowing if the timezone is ambiguous at other times.

This may be very frustrating, but it is sufficient to support one very common pattern when processing dates: parse and convert to UTC. This approach is recommended because it restricts all the complications with timezone handling to one place in the program - handling input. The rest of the code can process UTC values with no concerns about further errors.

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