Lesson 17
Reading and Writing Files
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Lesson 17
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Lesson objectives
1. Open files for reading, writing, or
appending data
2. Write data to a text file
3. Use the os module to manipulate
paths and pathnames
4. Use the pickle module to store
complex data types
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Files in Python
 Files are objects
 http://www.python.org/doc/2.5.2/lib
/bltin-file-objects.html
 Python file methods are wrappers
for the standard C stdio package
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File types
 Text file
 Contains ACSII or Unicode characters
 Can be created and read by most
applications
 Text 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.)
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File types
 Binary file
 Contain data coded in other formats
 Examples:
 JPEG images
 Audio or video clips
 Packed binary data from FORTRAN
 Matlab data files (.m files)
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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
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The filename argument
 Should be a string containing the
complete name of the file, including
the file extension
 NB: In MS Windows, most file extensions
are hidden in Windows Explorer
 Can include a partial or complete path
 Default path is the folder containing the
main script (.py file)
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File modes: reading a file
 'r'
 'rb'
read (text file)
read (binary file)
 Can read file contents but cannot
change file
 If file does not exist, raises
exception
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File modes: writing
 'w'
 'wb'
write (text file)
write (binary file)
 Create a new file
 Overwrites existing file if there is
one
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File modes: append
 'a'
 'ab'
append (text file)
append (binary file)
 Append data to (the end of) a file
 If file does not exist, creates a new
file
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File modes: mixed modes
 'r+'
read and write existing file
 If file does not exist, raises exception
 'a+'
read and write existing file
 Creates new file if one does not exist
 'w+'
read and write a new file
 Overwrites file if it already exists
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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
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End-of-line translations
 Unix and Linux (and Mac OS X)
 Use newline: \n
 DOS and Windows
 Use return + newline: \r\n
 Old Mac OSs
 Use return: \r
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End-of-line translations
 Python automatically translates
Windows EOLs when reading and
writing files on Windows
platforms
 When in text mode
 Not in binary mode
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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
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File read methods
 file.read()
 Read all data until EOF is reached and return as
a string object
 file.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
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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()
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File write methods
 file.write(str)
 Write a string to the file
 NB: 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 file
 NB: Does not add line separators, but this can
be done using the string join operator
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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()
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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()
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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
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File and directory commands
 os.getcwd()
 Returns the name of the current wording
directory as a string
 os.chdir(path)
 Changes the current working directory for
this process to path, a directory name
string
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File and directory commands
 os.listdir(path)
 Returns a list of names of all the entries
in the directory path
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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
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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
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The pickle module
 Pickling
 the process whereby a Python object
hierarchy is converted into a byte
stream
 Unpickling
 the inverse operation, whereby a
byte stream is converted back into
an object hierarchy
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The pickle module
 pickle.dump(obj, file)
 Write a pickled representation of obj to
the open file object file
 pickle.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
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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()
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Example: pickling.py
seq = []
print seq
f = open('temp.pk', 'r')
seq = pickle.load(f)
print seq
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