Python Programming Files, Lecture notes of Computer Science

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Python Programming

Chapter 7 - Files

Python for Everybody

www.py4e.com

File Processing

A text file can be thought of as a sequence of lines From [email protected] Sat Jan 5 09:14:16 2008 Return-Path: Date: Sat, 5 Jan 2008 09:12:18 - 0500 To: [email protected] From: [email protected] Subject: [sakai] svn commit: r39772 - content/branches/ Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=

http://www.py4e.com/code/mbox-short.txt

Using open()

• 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 fhand = open('mbox.txt', 'r')

What is a Handle?

>>> fhand = open('mbox.txt')

>>> print(fhand)

<_io.TextIOWrapper name='mbox.txt' mode='r' encoding='UTF-8'>

The newline Character

• We use a special character called the “newline” to indicate when a line ends
• We represent it as \n in strings
• Newline is still one character - not two

>>> stuff = 'Hello\nWorld!'

>>> stuff

'Hello\nWorld!'

>>> print(stuff)

Hello

World!

>>> stuff = 'X\nY'

>>> print(stuff)

X

Y

>>> len(stuff)

File Processing

A text file can be thought of as a sequence of lines From [email protected] Sat Jan 5 09:14:16 2008 Return-Path: Date: Sat, 5 Jan 2008 09:12:18 - 0500 To: [email protected] From: [email protected] Subject: [sakai] svn commit: r39772 - content/branches/ Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=

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 file is a string in the sequence
• We can use the for statement to iterate through a sequence
• Remember - a sequence is an ordered set xfile = open('mbox.txt') for cheese in xfile: print(cheese)

Reading the Whole File

We can read the whole file (newlines and all) into a single string

> >> fhand = open('mbox-short.txt')

>>> inp = fhand.read()

>>> print(len(inp))

>>> print(inp[:20])

From stephen.marquar

Searching Through a File

We can put an if statement in our for loop to only print lines that meet some criteria

fhand = open('mbox-short.txt')

for line in fhand:

if line.startswith('From:') :

print(line)

• Each line from the file has a newline at the end
• The print statement adds a newline to each line What are all these blank lines doing here?

From: [email protected]\n

\n

From: [email protected]\n

\n

From: [email protected]\n

\n

From: [email protected]\n

\n

OOPS!

Searching Through a File (fixed)

• We can strip the whitespace from the right-hand side of the string using rstrip() from the string library
• The newline is considered “white space” and is stripped fhand = open('mbox-short.txt') for line in fhand: line = line.rstrip() if line.startswith('From:') : print(line)

From: [email protected]

From: [email protected]

From: [email protected]

From: [email protected]

Using in to Select Lines

We can look for a string anywhere in a line as our selection criteria fhand = open('mbox-short.txt') for line in fhand: line = line.rstrip() if not '@uct.ac.za' in line : continue print(line) From [email protected] Sat Jan 5 09:14:16 2008 X-Authentication-Warning: set sender to [email protected] using – f From: [email protected] Author: [email protected] From [email protected] Fri Jan 4 07:02:32 2008 X-Authentication-Warning: set sender to [email protected] using - f...

Prompt for

File Name

fname = input('Enter the file name: ') fhand = open(fname) 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: mbox-short.txt

There were 27 subject lines in mbox-short.txt