Python lec5

Swarup Kr Ghosh
MIS Group
IIM Calcutta
swarupg1@gmail.com

 Regular Expression
 File Input/output
 Operating System
 Time
7/21/2017Swarup Kr Ghosh 2

 Special sequence of characters that helps ‘match’ or
‘find’ ‘other strings or sets of strings’
 Regular expressions are widely used in UNIX world
 The module ‘re’ provides full support for Perl-like
regular expressions in Python
 The re module raises the exception ‘re.error’ if an error
occurs while compiling or using a regular expression

 This function attempts to match RE pattern to string with
optional flags
Syntax: re.match(pattern, string, flags=0)

 The re.match() returns a match object on success,
None on failure
 We would use group(num) or groups() of match
object to get matched expression

import re
Match= re.search(‘iig’,’called piiig’)
Match.group() #or match
‘iig’
import re
Match= re.search(‘igs’,’called piiig’)
Match.group() #or match
‘None type object has no attribute group’

import re
>>> line="Cats are smarter than dogs"
>>> matchObj=re.match(r'(.*) are (.*?) .*',line, re.M|re.I)
>>> if matchObj:
print("matchObj.group() :",matchObj.group())
print("matchObj.group(1) :",matchObj.group(1))
else:
print("No match!!")
matchObj.group() : Cats are smarter than dogs
matchObj.group(1) : Cats
matchObj.group(2) : smarter

 This function searches for first occurrence of RE pattern
within string with optional flags
 Syntax: re.search(pattern, string, flags=0)

 The re.search() returns a match object on success,
None on failure
 We would use group(num) or groups() function of match
object to get matched expression

>>> import re
>>> line="Cats are smarter than dogs"
>>> matchObj=re.search(r'(.*) are (.*?) .*',line, re.M|re.I)
>>> if matchObj:
print("matchObj.group() :",matchObj.group())
else:
print("No match!!")
matchObj.group() : Cats are smarter than dogs
matchObj.group(1) : Cats
matchObj.group(2) : smarter

 Python offers two different primitive operations
based on regular expressions:
 match() checks for a match only at the beginning
of the string
 search() checks for a match anywhere in the string

import re
line = "Cats are smarter than dogs";
matchObj = re.match( r'dogs', line, re.M|re.I)
if matchObj:
print ("match --> matchObj.group() : ", matchObj.group() )
else:
print ("No match!!“)
matchObj = re.search( r'dogs', line, re.M|re.I)
if matchObj:
print ("search --> matchObj.group() : ", matchObj.group())
else:
print ("Not found!!“)
No match!!
search --> matchObj.group() : dogs

 find() determines if string str occurs in string, or in a
substring of string if starting index beg and ending
index end are given
 Syntax: str.find(str, beg=0 end=len(string))
def Find(pat,text):
match=re.search('pat','text')
if match:
print(match.group())
else:
print("Not Found")

>>> import re
>>> def find(pat,text):
match=re.search(pat,text)
if match:
print(match.group())
else:
print("Not Found")
>>> find('iiig','called piiig')
iiig
>>> find('ig','called piiig')
ig
>>> find('igs','called piiig')
Not Found

 Any special character written as . or *
.(dot) any character
w  word character
d  digit
s white space
S  non white space

>>> find(':www','c.lled:cat much better:xyz')
:cat
>>> find(':ddd','c.lled:cat123xyz')
Not Found
>>> find('ddd','c.lled:cat123xyz')
123
>>> find(':ddd','c.lled:cat:123xyz')
:123
>>> find('ds+ds+d','1 2 3')
1 2 3
>>> find(':w+s+w+','c.iied :cat123xyz better :xyz')
:cat123xyz better
>>> find(':.+','c.iied :cat123xyz better :xyz')
:cat123xyz better :xyz
>>> find('[w.]+@[w.]+','blah nick.p@gmail.com yahoo@')
nick.p@gmail.com
>>> find('w+@w+','blah nick.p@gmail.com yahoo@')
p@gmail
>>> m=re.search('[w.]+@[w.]+','blah nick.p@gmail.com yahoo@')
>>> m
<_sre.SRE_Match object; span=(5, 21), match='nick.p@gmail.com'>
>>> m.group()
'nick.p@gmail.com'

re.findall('[w.]+@[w.]+','blah nick.p@gmail.com yahoo@in')
['nick.p@gmail.com', 'yahoo@in']
re.findall('([w.]+)@([w.]+)','blah nick.p@gmail.com yahoo@in')
[('nick.p', 'gmail.com'), ('yahoo', 'in')]

 replace() method replaces all occurrences of the
RE pattern in string with repl, substituting all
occurrences unless max provided
 This method would return modified string
 Syntax:
◦ str.replace(old, new[, max])
◦ re.sub(pattern, repl, string, max=0)

import re
phone = "2004-959-559 “ #This is Phone Number
num = re.sub(r'#.*$', "", phone) #Delete comments
print ("Phone Num : ", num)
#Remove anything other than digits
num = re.sub(r'D', "", phone)
print "Phone Num : ", num
Phone Num : 2004-959-559
Phone Num : 2004959559
str = "this is string example....wow!!! this is really string"
print(str.replace("is", "was"))
print(str.replace("is", "was", 3))
thwas was string example....wow!!! thwas was really string thwas
was string example....wow!!! thwas is really string

 Reading Keyboard Input:
 raw_input()
◦ reads one line from standard input and returns it as a
string (removing the trailing newline)
 input()
◦ equivalent to raw_input, except that it assumes the input
is a valid Python expression and returns the evaluated
result to you

raw_input():
str = raw_input("Enter your input: ")
Print("Received input is : ", str)
Enter your input: Hello Python
Received input is : Hello Python
input():
str = input("Enter your input: ")
print ("Received input is : ", str)
Enter your input: [x*5 for x in range(2,10,2)]
Recieved input is : [10, 50, 10]

 Syntax:
file object = open(file_name [, access_mode][, buffering])
 file_name: The file_name argument is a string value that
contains the name of the file
 access_mode: The access_mode determines the mode in
which the file has to be opened, i.e., read, write, append
 buffering: If the buffering value is set to 0, no buffering, if the
buffering value is 1, line buffering, if you specify the buffering
value as an integer greater than 1, then buffering action will
be performed with the indicated buffer size

Modes Descriptions
R Opens file readonly
Rb Reading in binary format
r+ Both reading and writing
rb+ Both reading and writing in bianry format
W Write only (if file does not exist, create a new file for writing)
Wb Write only in binary format (if file does not exist, create)
w+ Both writing and reading
wb+ Both writing and reading in binary format
A Open a file for appending (if file does not exist, create)
Ab Appending in binary format
a+ Both appending and reading
ab+ Both appending and reading in binary format

 file.closed():
◦ Return true if file is closed, false otherwise
 file.mode():
◦ Return access mode with which file was opened
 file.name():
◦ Returns name of file
 file.softspace():
◦ Return false if space explicity required with print, true
otherwise

 filename.close():
◦ ---to close a file
 filename.read():
◦ ---to read a file
 filename.write():
◦ ---to write a file

f= open(“wavelet.txt”,’w’)
f.close()
print(“Name of file:”,f.name)
print(“Closed or not:”,f.close)
print(“Opening mode:”,f.mode)
Name of file: wavelet2.txt
Closed or not: true
Opening mode: w

f= open(“wavelet2.txt”, ‘w’)
f.write(“Python is a great programming language”)
f.close()
o/p:
The message have been written in wavelet2.txt
f= open(“wavelet2.txt”, ‘r’)
strf=f.read(10)
f.close()
o/p:
1st ten characters of the file wavelet2.txt

 filename.read():
◦ ---show whole file
 filename.readline():
◦ ---show first line of the file

def cat(file name):
f= open(filename, ‘r’)
for line in f:
print(line)
return
def cat(file name):
lines=f.read()
return
def cat(file name):
f.readline()
return

# Read a basic CSV file
f = open(“scmods.csv”,”rb”)
for r in csv.reader(f):
print (r)
# Write a basic CSV file
data = [
[‘Blues’,’Elwood’,’1060 W Addison’,’Chicago’,’IL’,’60613’ ],
[‘McGurn’,’Jack’,’4802 N Broadway’,’Chicago’,’IL’,’60640’ ],
]
f = open(“address.csv”,”wb”)
w = csv.writer(f)
w.writerows(data)
f.close()

 Python os module provides methods that help you
perform file-processing operations, such as
renaming and deleting files
 rename()
◦ Syntax:
◦ os.rename(current_file_name, new_file_name)
 remove():
◦ Syntax:
◦ os.remove(file_name)

import os
os.rename(test1.txt,test2.txt)
Os.remove(test2.txt)

import os
os.mkdir(“test”) #create a directory
os.chdir(“c:python34Tools”) #change directory
os.getcwd() #location of current directory
os.rmdir(“test”) #remove directory

import os
f=open("wavelet1.txt",'r')
str1=f.read() #read whole file
print("read string is:",str1)
f.close()
print("Name of file:",f.name)
print("closed or not:",f.closed)
print("opening mode:",f.mode)
print(str1) #print whole file

import os
f=open("wavelet1.txt",'r')
print("name of the file:",f.name)
for index in range(5):
line=f.next()
print("Line No %d - %s",%(index,line))
f.close()
o/p:
Line No 0 – This is the 1st line
Line No 1 – This is the 2nd line
………

>>> import time
>>> ticks=time.time()
>>> print("no of ticks since 12:00am,jan 1, 1970:",ticks)
no of ticks since 12:00am,jan 1, 1970: 1428682716.747197
#To get local current time:
>>> import time
>>> localtime=time.localtime(time.time())
>>> print("Local current time:",localtime)
Local current time: time.struct_time(tm_year=2015,
tm_mon=4, tm_mday=10, tm_hour=21, tm_min=53,
tm_sec=37, tm_wday=4, tm_yday=100, tm_isdst=0)

 Python CGI programming
 Python Database access
 Python Networking
 Python XML processing
 Python GUI programming

#!/usr/bin/env python
import numpy as np
import matplotlib.mlab as mlab
import matplotlib.pyplot as plt
mu, sigma = 100, 15
x = mu + sigma*np.random.randn(10000)
# the histogram of the data
n, bins, patches = plt.hist(x, 50, normed=1,
facecolor='green',
alpha=0.75)
# add a 'best fit' line
y = mlab.normpdf( bins, mu, sigma)
l = plt.plot(bins, y, 'r--', linewidth=1)
plt.xlabel('Smarts')
plt.ylabel('Probability')
plt.title(r'$mathrm{Histogram of IQ:} mu=100,
sigma=15$')
plt.axis([40, 160, 0, 0.03])
plt.grid(True)
plt.show()

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