Python MiniCourse

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Lesson 17Reading and Writing Files

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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Lesson objectives

• Open files for reading, writing, or appending
  data
• Write data to a text file
• Use the os module to manipulate paths and
  pathnames
• 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-obj
  ects.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' read (text file)
• 'rb' 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' write (text file)
• 'wb' write (binary file)
• Create a new file
• Overwrites existing file if there is one

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File modes append

• 'a' append (text file)
• 'ab' 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\t2.4f' (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\t2.4f' (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