Files and I/O

Reading and writing text from a file is fairly simple in Python.

Opening and Closing Files

3 modes for opening a file:

Append mode is similar to write mode, except that in append mode, any text that is written to the file is added to the end of any pre-existing text in the file.

To open a file:
myFileVar = open("myFileName", "theMode")

Example: To read from a file "myData.txt"
inFile = open("myData.txt", "r")

To close a file:

In order to read from a file, it must exist and have its permissions set so that it is readable.

Write vs. Append Mode:
If you open an existing file in write mode, any data in the file will be over-written. If you want to preserve the original contents of the file, use append mode instead.

Note: A file must be closed after you are finished reading or writing.

Reading a File

A small file can be read all at once with the read() method:

inFile = open("data.txt", "r")
fileContent =

For files that are too large to be read all at once, read it line by line with the method readline(). The readline() command will read an entire line, including the newline character.

inFile = open("input.txt", "r")
oneLine = inFile.readline()  # get first line in file

At any point, you can read the rest of the lines with the readlines() command, which returns a list of strings, one string per line - remember that the newline characters will be included in the strings.

If you want to read one line at a time from the file, you could also do this:

inFile = open("data.txt", "r")
for line in inFile:
    # do something with the line

Exercise: Write a program that opens a file for reading, and then for each line in the file, prints it twice in uppercase to the screen. (Hint: import the string library and use the string.upper() function).

Writing and Appending to a File

The write() command adds text to a file. This method takes a string as its parameter.

You must explicitly insert newline characters wherever you want a line break.

outFile = open("data2.txt", "w")
outFile.write("My first line\n")
outFile.write("Some more text")
outFile.write("And still more")

File data2.txt:
My first line
Some more textAnd still more

Exercise: Write a program that writes the numbers 1 to 10, one per line, in a new file.  Remember: The write() command takes a string as its parameter.