Reading and writing files, Study notes of Advanced Computer Programming

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Reading and writing files

We can interact with files in three modes

1. read mode ("r"):

Read data from file into python variables. File

remains unchanged.

2. write mode ("w"):

Write data from python variables to file. Previous file

contents is overwritten.

3. append mode ("a"):

Add data to the end of an existing file.

We interact with files via file handles

• A file handle is a python object that allows us to interact with a

file.

• Example 1. Open file for reading:

the open() function opens the file

and returns a handle

file_handle = open("file.txt", "r") # open in 'r' mode contents = file_handle.read() # reads the entire file file_handle.close() # always close at the end

We interact with files via file handles

• A file handle is a python object that allows us to interact with a

file.

• Example 2. Open file for writing:

the open() function opens the file

and returns a handle

file_handle = open("file.txt", "w") # open in 'w' mode

write one line

file_handle.write("New file contents.\n") file_handle.close() # always close at the end

The newline character ("\n")

In [1]: s = "String with newline.\n"

print() adds an additional "\n"

print(s) print(s) Out[1]: String with newline. String with newline.

The newline character ("\n")

In [1]: s = "String with newline.\n"

we can use end to suppress 2nd "\n":

print(s, end='') print(s, end='') Out[1]: String with newline. String with newline.

The newline character ("\n")

In [1]: s = "String with newline.\n"

using both eliminates all newlines:

print(s.rstrip(), end='') print(s.rstrip(), end='') Out[1]: String with newline.String with newline.

The newline character ("\n")

Unlike print(), the .write() function does not add a "\n".

There are multiple ways to read a file

1. Read the whole file at once:

contents = file_handle.read()

# The variable contents now holds the

# entire file in one long string.

There are multiple ways to read a file

2. Read the file into a list of lines:

lines = file_handle.readlines()

# The variable lines now holds a list of

# strings, each corresponding to one line

# in the file

Let Python close the file for you:

The with statement

traditional open – work with file – close sequence

file_handle = open("file.txt", "r") contents = file_handle.read() file_handle.close()

alternative form using with

with open("file.txt", "r") as file_handle: contents = file_handle.read()

the file is closed automatically when the indented

code-block ends.