Python Mini-Course University of Oklahoma Department of Psychology Lesson 17 Reading and Writing Files 5/10/09 Python Mini-Course: Lesson 17 1

Python Mini-CourseUniversity of Oklahoma

Department of Psychology

Lesson 17Reading and Writing Files

5/10/09Python Mini-Course: Lesson 171

Lesson objectives

1. Open files for reading, writing, or appending data

2. Write data to a text file3. Use the os module to manipulate

paths and pathnames4. Use the pickle module to store

complex data types


Files in Python

Files are objectshttp://www.python.org/doc/2.5.2/lib/

bltin-file-objects.htmlPython file methods are wrappers for the standard C stdio package


File types

Text fileContains ACSII or Unicode charactersCan be created and read by most

applicationsText editors (Notepad, SimpleText, etc.)IDEs (IDLE, SPE, Eclipse, etc.)Word processors (MS Word, etc.)Spreadsheet programs (Excel, etc.)Other apps (SAS, SPSS, R, Mathmatica, etc.)


File types

Binary fileContain data coded in other formatsExamples:

JPEG imagesAudio or video clipsPacked binary data from FORTRANMatlab data files (.m files)


The open statement

Returns a file object for access with file methods

Syntax fid = open(filename, mode)

where fid is the name of the file object


The filename argument

Should be a string containing the complete name of the file, including the file extensionNB: In MS Windows, most file extensions

are hidden in Windows ExplorerCan include a partial or complete

pathDefault path is the folder containing the

main script (.py file)5/10/09Python Mini-Course: Lesson 177

File modes: reading a file

'r' read (text file)'rb' read (binary file)

Can read file contents but cannot change file

If file does not exist, raises exception


File modes: writing

'w' write (text file)'wb' write (binary file)

Create a new fileOverwrites existing file if there is one


File modes: append

'a' append (text file)'ab' append (binary file)

Append data to (the end of) a fileIf file does not exist, creates a new file


File modes: mixed modes

'r+' read and write existing fileIf file does not exist, raises exception

'a+' read and write existing fileCreates new file if one does not exist

'w+' read and write a new fileOverwrites file if it already exists


Note

Data transferred between files and your programs is represented as Python strings, even if it is binary data.String objects can contain character bytes of any value


End-of-line translations

Unix and Linux (and Mac OS X)Use newline: \n

DOS and WindowsUse return + newline: \r\n

Old Mac OSsUse return: \r


End-of-line translations

Python automatically translates Windows EOLs when reading and writing files on Windows platformsWhen in text modeNot in binary mode


Example: eol.py, win.txt, mac.txt

text_mode = [open('win.txt','r').read(),

open('mac.txt','r').read()]

print 'Text mode:'

print text_mode

binary_mode = [open('win.txt','rb').read(),

open('mac.txt','rb').read()]

print '\nBinary mode:'

print binary_mode


File read methods

file.read()Read all data until EOF is reached and return as

a string objectfile.readline()

Read one entire line from the file (keeps the trailing newline character) and return as a string object

file.readlines()Read until EOF using readline() and return a

list containing the lines thus read


Example: read.py

fin = open('win.txt', 'r')print fin.read()fin.seek(0)print fin.readline()fin.seek(0)print fin.readlines()fin.close()


File write methods

file.write(str)Write a string to the fileNB: Due to buffering, the string may not actually

show up in the file until the flush() or close() method is called

file.writelines(sequence)Write a sequence of strings to the fileNB: Does not add line separators, but this can be

done using the string join operator


Example: randnums.py

import random

fout = open('rand.txt', 'w')

fout.write('Number\n')

seq = []

for i in range(10):

s = '%2.4f' % (random.random())

seq.append(s)

fout.writelines('\n'.join(seq))

fout.close()


Example: randnums2.py

import random

fout = open('rand.txt', 'w')

fout.write('Index\tNumber\n')

seq = []

for i in range(10):

s = '%d\t%2.4f' % (i, random.random())

seq.append(s)

fout.write('\n'.join(seq))

fout.close()


The os module

Provides generic operating system (OS) support and a standard, platform-independent OS interface

Includes tools for environments, processes, files, shell commands, and much more

http://www.python.org/doc/2.5.4/lib/module-os.html


File and directory commands

os.getcwd()Returns the name of the current wording

directory as a stringos.chdir(path)

Changes the current working directory for this process to path, a directory name string


File and directory commands

os.listdir(path)Returns a list of names of all the entries

in the directory path


Portability constants

os.curdir()String for the current directory

os.pardir()String for the parent directory

os.sep()String used to separate directories

os.linesep()String used to terminate lines


The pickle module

Used to serialize and de-serialize a Python object structure

http://www.python.org/doc/2.5.4/lib/module-pickle.html


The pickle module

Picklingthe process whereby a Python object hierarchy is converted into a byte stream

Unpicklingthe inverse operation, whereby a byte stream is converted back into an object hierarchy


The pickle module

pickle.dump(obj, file)Write a pickled representation of obj to

the open file object filepickle.load(file)

Read a string from the open file object file and interpret it as a pickle data stream, reconstructing and returning the original object hierarchy


Example: pickling.py

import random, pickle

seq = []

for i in range(10):

s = '%d\t%2.4f' % (i, random.random())

seq.append(s)

print seq

f = open('temp.pk', 'w')

pickle.dump(seq, f)

f.close()


Example: pickling.py

seq = []

print seq

f = open('temp.pk', 'r')

seq = pickle.load(f)

print seq


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Python Mini-Course University of Oklahoma Department of Psychology Lesson 17 Reading and Writing Files 5/10/09 Python Mini-Course: Lesson 17 1