Reading Files

Unit – 5B

Python for Everybody: Chapter 7

 Software What
It is time to go find some
Next? Data to mess with!
Input Central
and Output Processing Files R
Devices Unit Us

Secondary
if x < 3: print Memory

Main From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

Memory Return-Path: <postmaster@collab.sakaiproject.org>
Date: Sat, 5 Jan 2008 09:12:18 -0500To:
source@collab.sakaiproject.orgFrom:
stephen.marquard@uct.ac.zaSubject: [sakai] svn commit: r39772 -
content/branches/Details:
http://source.sakaiproject.org/viewsvn/?view=rev&rev=39772
...
 File Processing
A text file can be thought of as a sequence of lines
From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008
Return-Path: <postmaster@collab.sakaiproject.org>
Date: Sat, 5 Jan 2008 09:12:18 -0500
To: source@collab.sakaiproject.org
From: stephen.marquard@uct.ac.za
Subject: [sakai] svn commit: r39772 - content/branches/

Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=39772

Check file shared: mbox-short.txt

 Opening a File
• Before we can read the contents of the file, we must tell Python
which file we are going to work with and what we will be doing
with the file

• This is done with the open() function

• open() returns a “file handle” - a variable used to perform

operations on the file

• Similar to “File -> Open” in a Word Processor

 Using open()
fhand = open('mbox.txt', 'r')
• handle = open(filename, mode)

• returns a handle use to manipulate the file

• filename is a string

• mode is optional and should be 'r' if we are planning to

read the file and 'w' if we are going to write to the file
 What is a Handle?
>>> fhand = open('mbox.txt')
>>> print(fhand)
<_io.TextIOWrapper name='mbox.txt' mode='r' encoding='UTF-8'>
 When Files are Missing
>>> fhand = open('stuff.txt')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or
directory: 'stuff.txt'
 The newline Character
>>> stuff = 'Hello\nWorld!'
>>> stuff
• We use a special character 'Hello\nWorld!'
called the “newline” to indicate >>> print(stuff)
when a line ends Hello
World!
• We represent it as \n in strings >>> stuff = 'X\nY'
>>> print(stuff)
X
• Newline is still one character - Y
not two >>> len(stuff)
3
 File Processing
A text file can be thought of as a sequence of lines

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

Return-Path: <postmaster@collab.sakaiproject.org>
Date: Sat, 5 Jan 2008 09:12:18 -0500
To: source@collab.sakaiproject.org
From: stephen.marquard@uct.ac.za
Subject: [sakai] svn commit: r39772 - content/branches/

Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=39772
 File Processing
A text file has newlines at the end of each line

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008\n

Return-Path: <postmaster@collab.sakaiproject.org>\n
Date: Sat, 5 Jan 2008 09:12:18 -0500\n
To: source@collab.sakaiproject.org\n
From: stephen.marquard@uct.ac.za\n
Subject: [sakai] svn commit: r39772 - content/branches/\n
\n
Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=39772\n
Reading Files in Python
 File Handle as a Sequence
• A file handle open for read can
be treated as a sequence of
strings where each line in the xfile = open('mbox.txt')
file is a string in the sequence for line in xfile:
print(line)
• We can use the for statement
to iterate through a sequence

• Remember - a sequence is an
ordered set
 Counting Lines in a File
fhand = open('mbox.txt')
• Open a file read-only count = 0
for line in fhand:
• Use a for loop to read each line count = count + 1
print('Line Count:', count)
• Count the lines and print out
the number of lines
$ python open.py
Line Count: 132045
 Reading the *Whole* File
>>> fhand = open('mbox-short.txt')
We can read the whole >>> inp = fhand.read()
file (newlines and all) >>> print(len(inp))
into a single string 94626
>>> print(inp[:20])
From stephen.marquar
 Searching Through a File

We can put an if statement in fhand = open('mbox-short.txt')

for line in fhand:
our for loop to only print lines
if line.startswith('From:') :
that meet some criteria print(line)
 OOPS!
From: stephen.marquard@uct.ac.za
What are all these blank
lines doing here? From: louis@media.berkeley.edu

From: zqian@umich.edu

From: rjlowe@iupui.edu
...
 OOPS!
What are all these blank From: stephen.marquard@uct.ac.za\n
lines doing here? \n
From: louis@media.berkeley.edu\n
• Each line from the file \n
has a newline at the end From: zqian@umich.edu\n
\n
• The print statement adds From: rjlowe@iupui.edu\n
a newline to each line \n
...
Searching Through a File (fixed)
fhand = open('mbox-short.txt')
• We can strip the whitespace for line in fhand:
from the right-hand side of line = line.rstrip()
if line.startswith('From:') :
the string using rstrip() from print(line)
the string library
From: stephen.marquard@uct.ac.za
• The newline is considered
From: louis@media.berkeley.edu
“white space” and is From: zqian@umich.edu
stripped From: rjlowe@iupui.edu
....
 Skipping with continue
fhand = open('mbox-short.txt')
We can conveniently for line in fhand:
skip a line by using the line = line.rstrip()
if not line.startswith('From:') :
continue statement continue
print(line)
 Using in to Select Lines
fhand = open('mbox-short.txt')
We can look for a string for line in fhand:
anywhere in a line as our line = line.rstrip()
if not '@uct.ac.za' in line :
selection criteria continue
print(line)

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

X-Authentication-Warning: set sender to stephen.marquard@uct.ac.za using –f
From: stephen.marquard@uct.ac.za
Author: stephen.marquard@uct.ac.za
From david.horwitz@uct.ac.za Fri Jan 4 07:02:32 2008
X-Authentication-Warning: set sender to david.horwitz@uct.ac.za using -f...
fname = input('Enter the file name: ')
fhand = open(fname)
count = 0
Prompt for
for line in fhand:
if line.startswith('Subject:') :
count = count + 1
File Name
print('There were', count, 'subject lines in', fname)

Enter the file name: mbox.txt

There were 1797 subject lines in mbox.txt

Enter the file name: mbox-short.txt

There were 27 subject lines in mbox-short.txt
 fname = input('Enter the file name: ')
try:
Bad File fhand = open(fname)
except:
print('File cannot be opened:', fname)

Names quit()

count = 0
for line in fhand:
if line.startswith('Subject:') :
count = count + 1
print('There were', count, 'subject lines in', fname)

Enter the file name: mbox.txt

There were 1797 subject lines in mbox.txt

Enter the file name: na na boo boo

File cannot be opened: na na boo boo
 Summary
• Secondary storage • Searching for lines

• Opening a file - file handle • Reading file names

• File structure - newline character • Dealing with bad files

• Reading a file line by line with a

for loop
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