Summary of 12 basic knowledge points of Python language

Of 12 kinds of commonly used in python programming basics: regular expression substitution, directory traversal method, list to sort by column, heavy, dictionaries, dictionaries, lists, string transfers, object operation time, the command-line argument parsing (getopt), print formatted output, hexadecimal conversion, python call system command or script, read and write python files.

1. Regular expression substitution

Goal: replace overview.gif in the string line with another string

>>> line = '<IMG ALIGN="middle" SRC='#'" />'

>>> mo=re.compile(r'(?<=SRC=)"([w+.]+)"',re.I) 
>>> mo.sub(r'"1****"',line)

'<IMG ALIGN="middle" SRC='#'" /span>
>>> mo.sub(r'replace_str_1',line)

'<IMG ALIGN="middle" replace_str_overview.gif BORDER="0" ALT="">'< /span>
>>> mo.sub(r'"testetstset"',line)

'<IMG ALIGN="middle" SRC='#'" /span>

Note: where \1 is the matched data and can be directly referenced in this way

2. Traverse the directory method

At some point, we need to traverse a directory to find a specific list of files, which can be easily traversed using the os.walk method

import os

fileList = []

rootdir = "/data"

for root, subFolders, files in os.walk(rootdir):

if '.svn' in subFolders: subFolders.remove('.svn')  #  Exclude specific directories 

for file in files:

  if file.find(".t2t") != -1:#  Find a file with a particular extension 

      file_dir_path = os.path.join(root,file)

      fileList.append(file_dir_path) 
print fileList

3. List sort

If each element of the list is a tuple, we want to sort it according to a column of the tuple
In the following example, we sorted the tuple by the second and third columns of the tuple in reverse order (reverse=True).

>>> a = [('2011-03-17', '2.26', 6429600, '0.0'), ('2011-03-16', '2.26', 12036900, '-3.0'),

 ('2011-03-15', '2.33', 15615500,'-19.1')]

>>> print a[0][0]

2011-03-17

>>> b = sorted(a, key=lambda result: result[1],reverse=True)

>>> print b

[('2011-03-15', '2.33', 15615500, '-19.1'), ('2011-03-17', '2.26', 6429600, '0.0'),

('2011-03-16', '2.26', 12036900, '-3.0')]

>>> c = sorted(a, key=lambda result: result[2],reverse=True)

>>> print c

[('2011-03-15', '2.33', 15615500, '-19.1'), ('2011-03-16', '2.26', 12036900, '-3.0'),

('2011-03-17', '2.26', 6429600, '0.0')]

4. List deduplication (list uniq)

If you need to remove duplicate elements from a list, use the following method

>>> lst= [(1,'sss'),(2,'fsdf'),(1,'sss'),(3,'fd')]

>>> set(lst)

set([(2, 'fsdf'), (3, 'fd'), (1, 'sss')])

>>>

>>> lst = [1, 1, 3, 4, 4, 5, 6, 7, 6]

>>> set(lst)

set([1, 3, 4, 5, 6, 7])

5. Dict sort

Generally, we sort by the key of the dictionary, but if we want to sort by the value of the dictionary, we use the following method

>>> from operator import itemgetter

>>> aa = {"a":"1","sss":"2","ffdf":'5',"ffff2":'3'}

>>> sort_aa = sorted(aa.items(),key=itemgetter(1))

>>> sort_aa

[('a', '1'), ('sss', '2'), ('ffff2', '3'), ('ffdf', '5')]

6, dictionaries, lists, strings to each other

The following is the generated database connection string, converted from the dictionary to the string

>>> params = {"server":"mpilgrim", "database":"master", "uid":"sa", "pwd":"secret"}

>>> ["%s=%s" % (k, v) for k, v in params.items()]

['server=mpilgrim', 'uid=sa', 'database=master', 'pwd=secret']

>>> ";".join(["%s=%s" % (k, v) for k, v in params.items()])

'server=mpilgrim;uid=sa;database=master;pwd=secret'

The following example converts a string to a dictionary

>>> a = 'server=mpilgrim;uid=sa;database=master;pwd=secret'

>>> aa = {}

>>> for i in a.split(';'):aa[i.split('=',1)[0]] = i.split('=',1)[1]

...

>>> aa

{'pwd': 'secret', 'database': 'master', 'uid': 'sa', 'server': 'mpilgrim'}

7. Time object operation

Converts the time object to a string

>>> import datetime

>>> datetime.datetime.now().strftime("%Y-%m-%d %H:%M")

  '2011-01-20 14:05' 

Time scale comparison

>>> import time

>>> t1 = time.strptime('2011-01-20 14:05',"%Y-%m-%d %H:%M")

>>> t2 = time.strptime('2011-01-20 16:05',"%Y-%m-%d %H:%M")

>>> t1 > t2

  False

>>> t1 < t2

  True 

Time difference value calculation, calculate 8 hours ago time

>>> datetime.datetime.now().strftime("%Y-%m-%d %H:%M")

  '2011-01-20 15:02'

>>> (datetime.datetime.now() - datetime.timedelta(hours=8)).strftime("%Y-%m-%d %H:%M")

  '2011-01-20 07:03' 

Converts a string to a time object

>>> endtime=datetime.datetime.strptime('20100701',"%Y%m%d")

>>> type(endtime)

  <type 'datetime.datetime'>

>>> print endtime

  2010-07-01 00:00:00 

Format the output from 1970-01-01 00:00:00 UTC to the current number of seconds    

>>> import time

>>> a = 1302153828

>>> time.strftime("%Y-%m-%d %H:%M:%S",time.localtime(a))

  '2011-04-07 13:23:48'

8. Command line parameter analysis (getopt)

Usually when you write some daily operations scripts, you need to enter different command-line options to achieve different functions according to different conditions
The getopt module in Python provides a good implementation of command line parameter parsing, the following distance. See the following program:

#!/usr/bin/env python

# -*- coding: utf-8 -*-

import sys,os,getopt

def usage():

print '''''

Usage: analyse_stock.py [options...]

Options:

-e : Exchange Name

-c : User-Defined Category Name

-f : Read stock info from file and save to db

-d : delete from db by stock code

-n : stock name

-s : stock code

-h : this help info

test.py -s haha -n "HA Ha"

'''
try:

opts, args = getopt.getopt(sys.argv[1:],'he:c:f:d:n:s:')

except getopt.GetoptError:

usage()

sys.exit()

if len(opts) == 0:

usage()

sys.exit() 
for opt, arg in opts:

if opt in ('-h', '--help'):

  usage()

  sys.exit()

elif opt == '-d':

  print "del stock %s" % arg

elif opt == '-f':

  print "read file %s" % arg

elif opt == '-c':

  print "user-defined %s " % arg

elif opt == '-e':

  print "Exchange Name %s" % arg

elif opt == '-s':

  print "Stock code %s" % arg

elif opt == '-n':

  print "Stock name %s" % arg 
sys.exit()

9. Print formats output

9.1 format the output string
Intercepts the string output. The following example will output only the first three letters of the string

>>> str="abcdefg"

>>> print "%.3s" % str

  abc

Output as fixed width, insufficient to use space completion, the following example output width is 10

>>> str="abcdefg"

>>> print "%10s" % str

     abcdefg

Intercepts a string and outputs it in a fixed width

>>> str="abcdefg"

>>> print "%10.3s" % str

         abc

Floating-point type data bits reserved

>>> import fpformat

>>> a= 0.0030000000005

>>> b=fpformat.fix(a,6)

>>> print b

  0.003000

Round round round round round round round round round round round round

>>> from decimal import *

>>> a ="2.26"

>>> b ="2.29"

>>> c = Decimal(a) - Decimal(b)

>>> print c

  -0.03

>>> c / Decimal(a) * 100

  Decimal('-1.327433628318584070796460177')

>>> Decimal(str(round(c / Decimal(a) * 100, 2)))

  Decimal('-1.33')

9.2 conversion of base

Sometimes you need to do different base conversions, see the following example (%x hexadecimal,%d decimal,%o decimal)

>>> num = 10

>>> print "Hex = %x,Dec = %d,Oct = %o" %(num,num,num)

  Hex = a,Dec = 10,Oct = 12

Python calls system commands or scripts

The system command is invoked with os.system(), and the output and return values cannot be obtained in the program

>>> import os

>>> os.system('ls -l /proc/cpuinfo')

>>> os.system("ls -l /proc/cpuinfo")

  -r--r--r-- 1 root root 0  3 month  29 16:53 /proc/cpuinfo

  0 

The system command is called with os.popen(), and the output of the command can be obtained in the program, but the return value of the execution cannot be obtained

>>> out = os.popen("ls -l /proc/cpuinfo")

>>> print out.read()

  -r--r--r-- 1 root root 0  3 month  29 16:59 /proc/cpuinfo  

The system command is called with orders.getstatusoutput (), and the return value of the command output and execution is available in the program

>>> import commands

>>> commands.getstatusoutput('ls /bin/ls')

  (0, '/bin/ls')

11. Python captures user Ctrl+C,Ctrl+D events

Sometimes, you need to capture the user's keyboard event in the program, such as CTRL +c exit, so you can exit the program more safely

try:

    do_some_func()

except KeyboardInterrupt:

    print "User Press Ctrl+C,Exit"

except EOFError:

    print "User Press Ctrl+D,Exit"

12. Python reads and writes files

Read the file to the list at one time, the speed is fast, suitable for the file is relatively small

track_file = "track_stock.conf"

fd = open(track_file)

content_list = fd.readlines()

fd.close()

for line in content_list:

    print line  

Line by line, slow, not enough memory to read the entire file (the file is too big)

fd = open(file_path)

fd.seek(0)

title = fd.readline()

keyword = fd.readline()

uuid = fd.readline()

fd.close()  

Differences between write and writelines    

Fd.write(STR) : write STR to a file. Write () does not add a new line after STR
Fd.writelines(content) : write the entire content to a file, as is, without adding anything after each line