python日期格式化与绘图

   画一个量随着时间变化的曲线是经常会遇到的需求，比如画软件用户数的变化曲线。画随时间变化的曲线主要用到的函数是matplotlib.pyplot.plot_date(date,num)。由于其第一个变量是datetime类型的，所以对于string类型的数据输入，首先需要进行格式化操作。

一、使用strptime进行string的格式化

1.1一个基本例子

      time=datetime.datetime.strptime('2014-12-28 13:49:30','%Y-%m-%d %H:%M:%S')

   　print time

      2014-12-28 13:49:30

1.2常见的具体格式化对应关系

   常见的对应关系如下表所示(2014-12-28 13:49:30)，更多可以参见python对时间日期做格式化 - 走到天亮 - 博客园

Year		Month		Day	Hour		Minute	Second
%y	%Y	%m	%b	%d	%H	%I	%M	%S
14	2014	12	Dec	28	13	01	49	30

二、一个具体的画图例子

统计windows下软件首次安装的数目并画出散点图(按日统计)和柱状图(按年统计)

import _winreg
import datetime
import matplotlib.dates
import matplotlib.pyplot 
import numpy as np
print "import winreg done!"

key = _winreg.OpenKey(_winreg.HKEY_LOCAL_MACHINE,r"SoftwareMicrosoftWindowsCurrentVersionUninstall")
#key = _winreg.OpenKey(_winreg.HKEY_CURRENT_USER,r"SoftwareMicrosoftWindowsCurrentVersionExplorer")
print key
all_software=[]
k=0
try:
    i=0    
    while 1:
        subkey_string=_winreg.EnumKey(key,i)
        #print subkey_string
        try:
            j=0
            temp={}
            get_installDate=0
            #some subkey may not have installdate subkey and value
            while 1:
                #subkey=_winreg.OpenKey(key,"InstallDate")
                subkey=_winreg.OpenKey(key,subkey_string)
                name, value, type_value = _winreg.EnumValue(subkey,j)
                #print repr(name),repr(value)
                #print "name is "+name
                if name=="DisplayName":
                    #print "Name equals DisplayName"
                    temp["DisplayName"]=value
                if name=="InstallDate":
                    #print "Name equals installdate"
                    temp["InstallDate"]=value
                    get_installDate=1
                j+=1
        except WindowsError:
            #print 'interruptted j='+str(j)
            if get_installDate:
                all_software.append(temp)
        #print repr(name)
        #print value
        i +=1
except WindowsError:
    print
for item in range(len(all_software)):
    all_software[item]["InstallDate"]=datetime.datetime.strptime(all_software[item]["InstallDate"].encode('ascii','ignore'),'%Y%m%d')
count={}
bar_by_year={}
bar_by_year['2012']=0
bar_by_year['2013']=0
bar_by_year['2014']=0
for item in all_software:
    if count.has_key(item["InstallDate"]):
        count[item["InstallDate"]]+=1
    else:
        count[item["InstallDate"]]=1
    bar_by_year[str(item['InstallDate'].year)]+=1
xvalues=[]
yvalues=[]
for key,value in count.iteritems():
    xvalues.append(key)
    yvalues.append(value)
xvalues=matplotlib.dates.date2num(xvalues)
matplotlib.pyplot.figure(1)
matplotlib.pyplot.plot_date(xvalues,yvalues)
matplotlib.pyplot.figure(2)
xvalues_2=np.arange(3)
yvalues_2=bar_by_year.values()
fig2=matplotlib.pyplot.bar(xvalues_2,yvalues_2,0.5)
matplotlib.pyplot.xticks(xvalues_2+0.25, ['2012','2013','2014'], rotation='vertical')
matplotlib.pyplot.show()

#value, type = _winreg.QueryValueEx(key, "EnableAutoTray")

三、参考材料

[1]Python 操作注册表 - 无恙 - 博客园

[2]c++ - How can I enumerate/list all installed applications in Windows XP? - Stack Overflow

[3]winreg – Windows registry access — Python v3.0.1 documentation

[4]graph - plotting time in python with matplotlib - Stack Overflow

[5]沙湖王 | matplotlib绘图2

[6]Python图表绘制：matplotlib绘图库入门 - 蝶梦庄周 - 博客园

[7]ticks_and_spines example code: ticklabels_demo_rotation.py — Matplotlib 1.4.2 documentation

[8]api example code: barchart_demo.py — Matplotlib 1.4.2 documentation