File_handling.md

File handling

• open function
• Reading files
• Writing to files
• Inplace editing with fileinput

open function

• syntax: open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)

>>> import locale
>>> locale.getpreferredencoding()
'UTF-8'

• We'll be seeing these modes in this chapter
  • r open file for reading
  • w open file for writing
  • a open file for appending
• default is text mode, so passing 'r' and 'rt' are equivalent
  • for binary mode, it would be 'rb', 'wb' and so on
• locale.getpreferredencoding() gives default encoding being used
• Python docs - open
• Python docs - standard encodings

Reading files

#!/usr/bin/python3

# open file, read line by line and print it
filename = 'hello_world.py'
f = open(filename, 'r', encoding='ascii')

print("Contents of " + filename)
print('-' * 30)
for line in f:
    print(line, end='')

f.close()

# 'with' is a simpler alternative, automatically handles file closing
filename = 'while_loop.py'

print("\n\nContents of " + filename)
print('-' * 30)
with open(filename, 'r', encoding='ascii') as f:
    for line in f:
        print(line, end='')

• default encoding is usually 'UTF-8', use 'ascii' where applicable
• using with and file handle name as f is usual convention

$ ./file_reading.py
Contents of hello_world.py
------------------------------
#!/usr/bin/python3

print("Hello World")


Contents of while_loop.py
------------------------------
#!/usr/bin/python3

# continuously ask user input till it is a positive integer
usr_string = 'not a number'
while not usr_string.isnumeric():
    usr_string = input("Enter a positive integer: ")

If file doesn't exist

#!/usr/bin/python3

with open('xyz.py', 'r', encoding='ascii') as f:
    for line in f:
        print(line, end='')

• Error if file is not found

$ ./file_reading_error.py 
Traceback (most recent call last):
  File "./file_reading_error.py", line 3, in <module>
    with open('xyz.py', 'r', encoding='ascii') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'xyz.py'
$ echo $?
1

• read entire file content as single string using read()

>>> f = open('hello_world.py', 'r', encoding='ascii')
>>> f
<_io.TextIOWrapper name='hello_world.py' mode='r' encoding='ascii'>
>>> print(f.read())
#!/usr/bin/python3

print("Hello World")

• read line by line using readline()

>>> f = open('hello_world.py', 'r', encoding='ascii')
>>> print(f.readline(), end='')
#!/usr/bin/python3
>>> print(f.readline(), end='')

>>> print(f.readline(), end='')
print("Hello World")

>>> f.close()
>>> print(f.readline(), end='')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file.

• read all lines of file as list using readlines()
  • note the plural form

>>> f = open('hello_world.py', 'r', encoding='ascii')
>>> all_lines = f.readlines()
>>> all_lines
['#!/usr/bin/python3\n', '\n', 'print("Hello World")\n']

• check if file is closed or not

>>> f = open('hello_world.py', 'r', encoding='ascii')

>>> f.closed
False

>>> f.close()
>>> f.closed
True

Writing to files

#!/usr/bin/python3

with open('new_file.txt', 'w', encoding='ascii') as f:
    f.write("This is a sample line of text\n")
    f.write("Yet another line\n")

• Use the write() method to print a string to files
• To add text to an existing file, use 'a' mode instead of 'w'

$ ./file_writing.py
$ cat new_file.txt 
This is a sample line of text
Yet another line

Inplace editing with fileinput

#!/usr/bin/python3

import fileinput

with fileinput.input(inplace=True) as f:
    for line in f:
        line = line.replace('line of text', 'line')
        print(line, end='')

• The files to be modified are specified as Command line arguments when the program is run
• Note that print function has to be used instead of f.write
• Since the line read every iteration already has newline character, end is assigned empty string
• Python docs - fileinput

$ ./inplace_file_editing.py new_file.txt
$ cat new_file.txt 
This is a sample line
Yet another line

$ # to change all files in current directory ending with .txt, use
$ ./inplace_file_editing.py *.txt

$ # stdin can also be passed as input, inplace gets disabled
$ echo 'a line of text' | ./inplace_file_editing.py
a line

• specifying filenames and backup extensions

# To specify filenames within the program itself
with fileinput.input(inplace=True, files=('file1.txt', 'file2.txt')) as f:

# To create backup of unmodified files, pass an extension to backup parameter
with fileinput.input(inplace=True, backup='.bkp') as f: