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2017-09-12 Python

Dates And Times

Dates and times in Python#

we will be using datetime a lot of the time

    import datetime

It has 4 modules we usually use:

  • date
  • time
  • datetime - lets us work with time and date at the same time
  • timedelta
  • timezone

Now#

    >>> datetime.datetime.now()
    datetime.datetime(2017, 9, 12, 9, 19, 45, 673933)

Replace#

Replace attributes of the datetime object

    >>> treehouse_start = treehouse_start.replace(hour=9, minute=0, second=0, microsecond=0)

Manually initialise a date#

    h_start = datetime.datetime(2014, 10, 15, 9)

Getting datetime difference#

When you subtract 2 datetimes you get a timedelta

    >>> datetime.datetime.now() - treehouse_start
    datetime.timedelta(0, 1383, 884597)
  • 0 - days
  • 1383 - seconds
  • 884597 - microseconds

You can check properties of a tiemdelta:

td = datetime.datetime.now() - treehouse_start
td.days
td.microseconds
td.seconds

Timedeltas#

Represent a gap of time

    datetime.timedelta(days=3)

You can also create negative 5 days

    datetime.timedelta(days=-5)

Then add that to a date:

    now = datetime.datetime.now()
    now - datetime.timedelta(days-5)

Just use the date or time

    now.date()
    now.time()

YOu can multiple timedeltas

    hour = datetime.timedelta(hour=1)
    workday = hour * 9

Other methods#

datetime.datetime.today() is different because you cannot control the timezone.

Combine a date and a time

    today = datetime.datetime.combine(datetime.date.today(), datetime.time())

Which creates a date at midnight

Find Day of the week#
    today.weekday()

Python weeks start on monday == 0

Find Posix Tiemstamp#
    today.timestamp()
Total seconds#

Get the total number of seconds in a timedelta

    time.total_seconds()

Converting times and dates#

Strftime#

strftime - Convert a date, time or datetime into a string formatted how we want

    now.strftime('%B %d')

Full cheatsheet of strftime python docs

Strptime#

Think of it as string parsed into time

It lets us make a datetime out of a string of a certain format

>>> birthday = datetime.datetime.strptime('2015-12-21', '%Y-%m-%d')
>>> birthday
datetime.datetime(2015, 12, 21, 0, 0)

Timezones#

A datetime object that knows its timezone is aware, those that don’t are naive

Creating a Timezone object#
    pacific = datetime.timezone(datetime.timedelta(hours=-8))
    eastern = datetime.timezone(datetime.timedelta(hours=-5))

The offset is from UTC

Create an aware datetime

    aware = datetime.datetime(2014, 4, 21, 9, tzinfo=pacific)

You can then change the timezone

    aware.astimezone(eastern)

You can also replace timezones

    import datetime

    naive = datetime.datetime(2015, 10, 21, 4, 29)

    timezone = datetime.timezone(datetime.timedelta(hours=-8))

    hill_valley = naive.replace(tzinfo=timezone)

Then move datetime to a new timezone:

    paris_tz = datetime.timezone(datetime.timedelta(hours=1))
    paris = hill_valley.astimezone(paris_tz)

Pytz#

    pip install pytz

Setting a timezone

    pacific = pytz.timezone('US/Pacific')
    utc = pytz.utc

Creating a datetime and makes aware:

    start = pacific.localize(datetime.datetime(2014, 4, 41, 9))
    start_eatern = start.astimezone(eastern)

localize is for naive timezones, we can’t localize again

Storing#

Store as UTC as it is easier to convert with daylight savings time DST

List all timezones#

pytz.all_timezones

Country timezones#

>>> pytz.country_timezones('za')
['Africa/Johannesburg']