Lesson 12 Notes: Working with Files

• Introduction
  • Python can be used to create programs that manipulate the files on a computer.
  • The os module contains functions for searching, listing, renaming, and deleting files.
  • The os.path module contains a few specialized functions for manipulating files
  • A number of functions for opening, reading, and writing to files are built into Python
• OS Functions: Just Looking
  • getcwd()
    • "get current working directory"
    • Returns the name of the current directory
  • listdir(path)
    • Returns a list of all files in a directory
  • chdir(path)
    • "change directory"
    • Move focus to a different directory
• OS Functions: Adding
  • mkdir(path)
    • "make directory"
    • Make a new directory with the given name
  • makedirs(path)
    • "make directories"
    • Makes a subdirectory and all required parent directories
    • Ex. makedirs("c:\\Python\newfiles\trash")
• OS Functions: Removing
  • remove(path)
    • Deletes a file
  • rmdir(path)
    • "remove directory"
    • Deletes an empty directory
  • removedirs(path)
    • Deletes a directory and everything in it
• OS Functions: Changing
  • rename(old,new)
    • Change the name of a file from "old" to "new"
  • renames(old,new)
    • Change name of file from old to new, changing directory names as needed
• os.path Functions: What is it?
  • exists(file)
    • Does the file exist?
  • isdir(path)
    • Is this a directory?
  • isfile(file)
    • Is this a file?
• Sample Code: Erase All
  import os, os.path
path = raw_input("folder to clean:")
os.chdir(path)
files = os.listdir(path)
print files
for file in files:
    if os.path.isfile(file):
       os.remove(file)
       print "erased", file
    elif os.path.isdir(file):
       os.removedirs(file)
       print "removed", file
    else:
      pass

• Openin' A File
  • open(filename, mode)
  • Modes (must be in quotes)
    • r = read
    • w = write (replaces exisiting data)
    • a = append (add to the end)
  • To be able to use opened file, give it a variable handle.
  • Example: openfile = open("filename.txt", "a")
• Readin' a File
  • file.read()
    • Read whole file as a string
  • file.readline()
    • Read a single line in as a string
  • file.readlines()
    • Read whole file, each line becomes string item in a list.
• Writin' To a File
  • file.write(string)
    • Write "string" to the file. How it is written will depend on mode the file is opened in.
  • file.writelines(list)
    • Write all the string items in a list to the file.
    • Each element in list will be on same line unless the element contains a newline character.
• Closin' Up
  • file.close()
    • Closes an opened file.
• Temp Files
  • Functions in tempfile module can be used to create and handle temporary files.
  • tempfile.maketmp() creates a temporary file that has a random, unused name. The tempfile is located in the default temporary folder.
• Sample Read/Write Script
  #Program to replace an old word with a new one
import string, tempfile, os
# Ask User for Information
filename = raw_input("Name of file: ")
find = raw_input("Search for: ")
replace = raw_input("Replace with: ")
#Open user file, read it, and close it
file = open(filename, "r")
text = file.readlines()
file.close()
#Edit information from user file
newtext =[]
for item in text:
    line = string.replace(item, find, replace)
    newtext.append(line)
#Create new file
newname=filename+".new"
newfile = open(newname, "w")
newfile.writelines(newtext)
newfile.close()
#Change names of files
oldfile=filename+".bak"
os.rename(filename, oldfile)
os.rename(newname, filename)
os.remove(oldfile)

• Exceptions
  • try and except can be used to catch errors in code.
  • try runs some commands or functions.
  • If a function fails or returns an error, the except commands are then run
  • Can be used to handle program killing crashes very nicely.
• Sample Try / Except
  def open_file():
    filename = raw_input("Enter File Name: ")
    try:
       file = open(filename)
    except:
       print "File not found."
       open_file()
    return file

• Review
  • What modules contain functions to manipulate files?
  • What are the different modes for opening a file?
  • What is the difference between readline() and readlines()? write(string) and writelines(list)?
  • Why would you use a try / except construction?