pandas 之 datetime 初识

import numpy as np
import pandas as pd

认识

Time series data is an impotant from of data in many different fields, such as finance, economics, ecology, neuroscience(神经学) and physics. Anything that is observed or measured at many points in time forms a time series.

Many time series are fixed frequency , which is to say that data points occur at regular intervals according to some rule, such as every 15 seconds, every 5 minutes, or once per month.

Time series can also be irregular(不规则的) without a fixed unit of time or offset between units. How you mark and refer to time series data depends on the application, and you may have one of the following:

• Timestamps, specific instants in time (时间戳)
• Fixed periods, such as the month January 2007 or the full year 2010 (时期)
• Intervals of time, indicated by a start and end timestamp. Periods can be thought of as special cases of intervals. (时间间隔)
• Experiment or elapsed time(试验时间逝去); each timestamp is a measure of time relative to a particular start time (e.g. the diameter(直径) of a cookie baking each second since being palced in the oven)

In this chapter, I am mainly concerned with time series in the first three categories, though many of the teachniques can applied to experimental time series where the index may be an integer or floating-point number indicating elapsed time from the start of the experiment. The simplest and most widely used kind of time series are those indexed by timestamp.

pandas also supports indexes based on timedeltas, which can be a useful way of representing experiment or elapsed time. We do not explore timedelta indexes in this book , but you can learn more in the pandas documenttaion.

pandas provides many buit-in time series tools and data algorithims. You can efficiently work with very large time series and easily slice and dice, aggregate, and resample(重采样) irrgular-and fixed-frequency time series. Some of these tools are especially useful financial and economics applications, but you could certainly use them to analyze server log, too.

The Pyhton standard library includes data types for date and time data, as well as calendar-related(日历相关) functionality. The datetime, time, calendar modules are the main places to start. the datetime.datetime type, or simply datetime, is widely used.

from datetime import datetime

now = datetime.now()

now

datetime.datetime(2019, 4, 27, 15, 3, 14, 103616)

now.year, now.month, now.day, now.hour, now.minute

(2019, 4, 27, 15, 3)

datetime stores(存储) both the date and time down to the microsecond timedelta reprecents the temporal(临时的) difference between two datetime objects:

"cj 特方便, 在时间相加上"

delta = datetime(2011, 1, 7) - datetime(2008, 6, 24, 8, 15)

delta

'cj 特方便, 在时间相加上'

datetime.timedelta(926, 56700)

delta.days, delta.seconds

(926, 56700)

You can add (or subtract) a timedelata or multiple thereof to a datetime object to yield a new shifted object:

from datetime import timedelta

start = datetime(2011, 1, 7)

"加12天"
start + timedelta(12)

'加12天'

datetime.datetime(2011, 1, 19, 0, 0)

" 减去24天"
start - 2*timedelta(12)

' 减去24天'

datetime.datetime(2010, 12, 14, 0, 0)

Table 11-1 summarizes the data types in the datetime module. While this chapter is mainly concerned with the data types in pandas and high-level time series manupulation, you may encounter the datetime-based types in many other places in Pyhton in the wild.

Type	Description
date	Store calendar date (year, month, day) using the Gregorian calendar
time	Store time of day as hours,minutes, seconds, and microseconds
datetime	Store both date and time
timedelta	Reprecents the difference between tow datetime values(as days,second..)
tzinfo	Base type for storing time zone infomation

String和Datetime间的转换

You can format datetime object and pandas Timestamp objects, which I'll introduce later, as strings using str or the strftime method, passing a format specification:

stamp = datetime(2011, 1, 3)

stamp
str(stamp)

datetime.datetime(2011, 1, 3, 0, 0)

'2011-01-03 00:00:00'

stamp.strftime('%Y-%m-%d')  # 四位数字的年

'2011-01-03'

stamp.strftime('%y-%m-%d')  # 2位数字的年

'11-01-03'

See Table 11-2 for a complete list of the format codes.

Type	Description
%Y	Four-digit year(4个数字的年)
%y	Two-digit year
%m	Two-dight month [01, 12]
%d	Two-dight day [01, 31]
%H	Hour(24-hour clock) [00, 23]
%I	Hour(12-hour clock) [00, 12])
%M	Two-dight minute [00, 59]
%S	Second [00, 61] (second 60, 61 acccount for leap second)
%w	Weekday as integer[0(Sundday), 6]
%U
%W
%z	UTC time zone offset as +HHMM or -HHMM; empty if time zone naive
%F	Shortcut for %Y-%m-%d (eg. 2012-4-8)
%D	Shortcut for %m/%d/%y (eg. 04/18/12)

You can use these same format codes to convert strings to dates using date time.strptime:

value = "2011-01-03"

datetime.strptime(value, '%Y-%m-%d')

datetime.datetime(2011, 1, 3, 0, 0)

datestrs = ['7/6/2011', '8/6/2011']

[datetime.strptime(x, '%m/%d/%Y') for x in datestrs]

[datetime.datetime(2011, 7, 6, 0, 0), datetime.datetime(2011, 8, 6, 0, 0)]

Datetime.strptime is a good way to parse a date with a know format. However, it can be a bit annoying to have to write a format spec each time, especially for common date formats.In this case, you can use the parse.parse method in the third-party dateutil package (this is installed automatically when you install pandas).

from dateutil.parser import parse

parse("2011-01-03")

datetime.datetime(2011, 1, 3, 0, 0)

parse("2011/01/03")

datetime.datetime(2011, 1, 3, 0, 0)

dateutil si capable of parsing most human-intelligble date representation:

parse('Jan 31, 1997, 10:45 PM')

datetime.datetime(1997, 1, 31, 22, 45)

In international locales, day appering before month is very common, so you can pass dayfirst=True to indicate this:

parse('6/12/2011', dayfirst=True)

datetime.datetime(2011, 12, 6, 0, 0)

pandas is generally oriented toward working with arrays of dates, whether used an axis index or a column in a DataFrame. The to_datetime method parses many different kinds of date representations. Standard date formats like ISO 8601 can be parsed very quickly:

datestrs = ['2011-07-06 12:00:00', '2011-08-06 00:00:00']

pd.to_datetime(datestrs)

DatetimeIndex(['2011-07-06 12:00:00', '2011-08-06 00:00:00'], dtype='datetime64[ns]', freq=None)

It also handles values that should be condidered missing (None, empty string. etc.):

idx = pd.to_datetime(datestrs + [None])

idx

DatetimeIndex(['2011-07-06 12:00:00', '2011-08-06 00:00:00', 'NaT'], dtype='datetime64[ns]', freq=None)

idx[2]

NaT

pd.isnull(idx)

array([False, False,  True])

NaT (Not a Time) is pandas's null value for timestamp data.

dateutil.parser is a useful but imperfect tool. Notably, it will recognize some strings as dates that you might prefer that it didn't for example. '42' will be parsed as the year 2042 with today's ccalendar date.

datetime objects also have a number of locale-specific formatting options for systems in other countries or languages. For example, the abbreviated(缩写) month names will be different on German or French systems compared with English systme. See Table 11-3 for a listing.

• %a Abbreviated weekday name
• %A Full weekday name
• %b 缩写月份的名字
• %B 全写月份
• %c Full date and time (eg. Tue 01 May 2012 04:20:57 PM)
• %p 包含AM or PM
• %x (eg. '05/01/2012')
• %X (eg. '04:24:12 PM')