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Highlights from this chapter include:

  • Python can read and write data in files using the built-in open() function.
  • Due to buffering, changes to a file may not be visible until the file is closed.
  • Newline characters need to be removed/added when reading/writing files.
  • Comma-separate-value (CSV) files are commonly used to represent data.
  • Exceptions that cause a program to terminate can be handled using try and except.
  • Raising an exception is an alternative way to return from a function when an error occurs.
Statement Description

file = open("myfile.txt")

Open a file for reading.

file = open("myfile.txt", 'w')

Open a file for writing.

file = open("myfile.txt", 'a')

Open a file for appending.

file.close()

Close a file after making changes.

data = file.read()

Read the entire contents of a file. The variable data is a string.

data = file.readline()

Read the next line of a file. The variable data is a string.

data = file.readlines()

Read all lines of a file. The variable data is a list of strings.

file.write("Have a nice day!\n")

Writes a line to a file. In contrast to print(), the write() function does not automatically append a newline.

file.write(["Line 1\n", "Line 2\n"])

Writes multiple lines to a file. As with write(), a newline is not automatically added at the end of each line.

try:
    # Statements
except:
    # Statements
Try to run statements that might raise an error. If any error is raised, run other statements.
try:
    # Statements
except ValueError:
    # Statements

Try to run statements that might raise a ValueError. If a ValueError is raised, run other statements.

try:
    # Statements
except ValueError as err:
    # Statements

Try to run statements that might raise a ValueError. If a ValueError is raised, run other statements. The variable err contains more details about the ValueError.

raise ValueError

Raises a ValueError with no specific error message.

raise ValueError("number is prime")

Raises a ValueError with the message "number is prime".

Table 14.3 Chapter 14 reference.
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