File Handling

Introduction

Python has built-in functions to perform operations on files, including reading and writing files. To perform a file operation, first, a file has to be opened. Then the file operation can be performed. Files are closed afterwards.

Opening a file

The built-in open() function opens and gains access to a file. It accesses a file by the specified filename (in the current directory) or a path to that file. A mode can also be specified for different purposes as follows:

'r': Opens the specified file for reading, and this is the default mode of operation. An error occurs if a file does not exist.
'w': Opens the specified file for writing. A new file will be created if it does not exist.
'a': Opens the specified file for appending. A new file will be created if it does not exist.
'x': Creates the specified file. An error occurs if the file exists.

As another option, the file could be specified to be one of the two types of mode:

t: Text mode, and this is the default mode.
b: Binary mode.

Reading a file

read(): read bytes as a string
readline(): read a line in the file as a string
readlines(): read all lines in the file as a list of strings (each line is a string)

Writing a file

write("a string is specified"): write a string as a line into the file.
writelines("a list of strings are specified"): write multiple strings, each string as a line, into a file.

Closing a file

The Python function close() will free up the memory spaces used by the file.

Read in a file and count the number of lines in it For example, count how many records in a file named "records.txt" with the content:

1. Jerry Smith, USA
2. Lily Tom, Canada
3. Peter Frank, France
4. Susan Yellen, USA
5. David Elli, Japan

It is first opened by open(), then each line is read in and counted at a time; after all lines are read in and counted, the file is closed, and the number of lines is displayed.

# opens records.txt in the current directory with read mode
records = open("records.txt", "r")

# the num_records variable acts as a counter and therefore, is initially set to 0
num_records = 0

for record in records:
    num_records += 1

# closes records.txt with close()
records.close()
print(num_records)

Output:

5

With statement

"with" statement prevents errors if the file is not closed properly.

Python syntax:

with open(<file name>, <mode>) as <a name representing the file object>:
    <use the attributes/methods of the named file object to do file hanling operations>

Write shopping items to a file by using "with" statement and write()

shopping_list = ["apples", "bananas", "green beans", "turkey"]

with open("shopping_memo.txt", "w") as f:
    for i in range(len(shopping_list)):
        item = shopping_list[i] + "\n"
        f.write(str(i+1) + ": " + item)

with open("shopping_memo.txt", "r") as f:
    lines = f.readlines()
    for line in lines:
        print(line.strip()) # strip() removes "\n"

The content in shopping_memo.txt after execution:

1: apples
2: bananas
3: green beans
4: turkey

Output:

1: apples
2: bananas
3: green beans
4: turkey