Python is recommended by many people as the first language to be learned now. Even in some universities, it is the primary language to be taught to CS/CE students. To master this languages, we should understand some basics of it. Here we summarize some basics about this language.
These basics include : string replacement with regular expression, traverse a directory, sort a list, remove duplication, dictionary ordering, dictionary, list, string conversion, date object manipulation, command line parameter parsing(getopt), formatted output, Call system commands or scripts with Python, Python file I/O.
1. String replacement with regular expression
Problem : Replace overview.gif with other string in line.
>>> line = '<IMG ALIGN="middle" SRC=\'#\'" /span> >>> 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 : \1 maps to the matched data.
2. Traverse a directory
Sometimes we need to traverse a directory to find some files we need, we can use os.walk to achieve this.
import os
fileList = []
rootdir = "/data"
for root, subFolders, files in os.walk(rootdir):
if '.svn' in subFolders: subFolders.remove('.svn') # Filter some specified directories
for file in files:
if file.find(".t2t") != -1:# Find files with specified extension name
file_dir_path = os.path.join(root,file)
fileList.append(file_dir_path)
print fileList
3. list sort
If each item in a list is a tuple, we need to sort the list according to a specified column in the tuple. We ca refer to below method. In the following example, we sort the list according to the 2nd and 3rd column and it's reverse sorted.
>>> 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. Remove list duplication
Below method can be used to remove duplication in a list:
>>> 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. Dictionary sort
Generally, we will sort a dictionary by its key. but if we want to sort the dictionary by its value, then we can use:
>>> 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. Dictionary,list,string conversion
Below example is to convert a dictionary to a 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'
Below is to convert 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. Date manipulation
Convert datetime to string
>>> import datetime
>>> datetime.datetime.now().strftime("%Y-%m-%d %H:%M")
'2011-01-20 14:05'
Datetime 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
Datetime difference
>>> 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'
Convert string to datetime
>>> endtime=datetime.datetime.strptime('20100701',"%Y%m%d")
>>> type(endtime)
<type 'datetime.datetime'>
>>> print endtime
2010-07-01 00:00:00
8. Command line argument parsing
When we write some monitoring scripts, we may need to pass different arguments for different conditions. In Python, we can use getopt to parse command line arguments.
#!/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. Formatted output
9.1 Formatted string output
Print part of the string, 3 characters will be printed out below:
>>> str="abcdefg" >>> print "%.3s" % str abc
Print the string with fixed width
>>> str="abcdefg"
>>> print "%10s" % str
abcdefg
Print part of string with fixed width
>>> str="abcdefg"
>>> print "%10.3s" % str
abc
Float number with fixed decimal point
>>> import fpformat >>> a= 0.0030000000005 >>> b=fpformat.fix(a,6) >>> print b 0.003000
Round float number
>>> 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 Number conversion
Sometimes we need to convert binary data to decimal data.
>>> num = 10 >>> print "Hex = %x,Dec = %d,Oct = %o" %(num,num,num) Hex = a,Dec = 10,Oct = 12
10. Python to call system commands or scripts
Use os.system() to call system commands, cannot get output and returned value
>>> import os
>>> os.system('ls -l /proc/cpuinfo')
>>> os.system("ls -l /proc/cpuinfo")
-r--r--r-- 1 root root 0 3月 29 16:53 /proc/cpuinfo
0
Use os.popen() to call system commands, can get output but cannot get returned value
>>> out = os.popen("ls -l /proc/cpuinfo")
>>> print out.read()
-r--r--r-- 1 root root 0 3月 29 16:59 /proc/cpuinfo
Use commands.getstatusoutput() to call system commands, can get output and returned value
>>> import commands
>>> commands.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
11. Use Python to capture Ctrl+C, Ctrl+D events
Sometimes we may need to capture key events.
try:
do_some_func()
except KeyboardInterrupt:
print "User Press Ctrl+C,Exit"
except EOFError:
print "User Press Ctrl+D,Exit"
12. Python file I/O
Read file to a list at once.
track_file = "track_stock.conf"
fd = open(track_file)
content_list = fd.readlines()
fd.close()
for line in content_list:
print line
Read file line by line. A bit slow.
fd = open(file_path) fd.seek(0) title = fd.readline() keyword = fd.readline() uuid = fd.readline() fd.close()
