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226 CHAPTER 10 ■ BATTERIES INCLUDED
The reason for the usefulness of the deque is that it allows appending and popping efficiently
at the beginning (to the left), as opposed to lists. As a nice side effect, you can also rotate the
elements (that is, shift them to the right or left, wrapping around the ends) efficiently. Deque
objects also have extend and extendleft methods, with extend working like the corresponding
list method, and extendleft working analogously to appendleft. Note that the elements in the
iterable used in extendleft will appear in the deque in reverse order.
time
The time module contains functions for, among other things, getting the current time, manipulating
times and dates, and reading dates from strings and formatting dates as strings. Dates
can be represented as either a real number (the seconds since 0 hours, January 1 in the “epoch,”
a platform-dependent year; for UNIX, it’s 1970), or a tuple containing nine integers. These integers
are explained in Table 10-5. For example, the tuple
(2002, 1, 21, 12, 2, 56, 0, 21, 0)
represents January 21, 2002, at 12:02:56, which is a Monday, and the 21st day of the year.
(No daylight savings.)
Table 10-5. The Fields of Python Date Tuples
Index Field Value
0 Year For example, 2000, 2001, and so on
1 Month In the range 1–12
2 Day In the range 1–31
3 Hour In the range 0–23
4 Minute In the range 0–59
5 Second In the range 0–61
6 Weekday In the range 0–6, where Monday is 0
7 Julian day In the range 1–366
8 Daylight Savings 0, 1, or –1
Some of these values need some explaining: The range for seconds is 0–61 to account for
leap seconds and double leap seconds. The Daylight Savings number is a Boolean value (true
or false), but if you use –1, mktime (a function that converts such a tuple to a timestamp measured
in seconds since the epoch) will probably get it right. Some of the most important functions in
the time module are described in Table 10-6.