10 File Handling

Saving and retrieving information is frequently required when working with computer programs. Python has methods to read and write files. In addition, there are some library that can facilitate working with specialized file types. For reading a file, a file handler need to be initated which is an object that can be use to work with file. A file handler incorporates information about the file along with its path and the mode in which the file is available for processing. The mode here refers to the manner in which the file would be opened i.e. for reading, writing, etc. The open function takes name of the file as an argument along with mode and returns the corresponding file handler. This file handler object is iteratable and can be considered as a list having of all the lines in the file. Once done with reading the contents for the file, the file handler must be closed using the close function.

pwd

'C:\\Users\\bioinfo guru\\OneDrive - bioinfo.guru\\Documents\\python_book'

10.1 Reading files

# %load test_file.txt
This is a test file for Python.
This file has .txt extension.

# test_file.txt should be present in the current working directory.

FH1 = open('test_file.txt','r')
#print(FH1)

#print(f"The file {FH1.name} is open in {FH1.mode} mode.")

# print all the lines in the file.
for x in FH1:
    print(x)
FH1.close()

This is a test file for Python.

This file has .txt extension.

Often it is useful to remove the newline character (\n) at the end of the lines since the print fuction add as newline by default. This can be achieved using the rstrip() function. This function without any argument remove the last character in the string and return a copy of the modified string.

10.2 Writing files

For writing content to a file the open function should be called with ‘w’ as the mode. When a file is opened in the write mode, a new file is created. In case there the file already exists then its contents are overwritten (without warning!). write function is used to write content to a file. We can also use print with file attribute to write to file instead of printing on screen.

FH_out = open('temp_file.txt', 'w')
FH_out.write("This is the first sentence.\n")
print("This is the second sentence.", file=FH_out)
FH_out.close()

# %load temp_file.txt
This is the first sentence.This is the second sentence.

FH2 = open('temp_file.txt','r')
for lines in FH2:
    print(lines)
FH2.close()

This is the first sentence.

This is the second sentence.

To append contents to an existing file, it should be opened with ‘a’ option instead of ‘w’.

FH_out = open('temp_file.txt','a')
FH_out.write("This is the third sentence.\n")
FH_out.write("This is the \t fourth sentence.\n")
FH_out.close()

FH3 = open('temp_file.txt','r')
for lines in FH3:
    lines = lines.rstrip('\n')
    print(lines)
FH3.close()

This is the first sentence.
This is the second sentence.
This is the third sentence.
This is the      fourth sentence.

The readlines function can be used to get a list having lines of the file as elements.

FH4 = open('temp_file.txt','r')
all_lines = FH4.readlines()
print(all_lines)
FH4.close()

['This is the first sentence.\n', 'This is the second sentence.\n', 'This is the third sentence.\n', 'This is the \t fourth sentence.\n']

10.3 The `with` keyword

We can also read and write file using the with keyword. Here some action are performed on the file object within the with block. This approach automatically closes the when the with block is over.

with open("temp123.txt", "w") as FH_OUT:
    print("Hello", file=FH_OUT, end=" ") 
    print("World!", file=FH_OUT)
print("Done")

Done

with open("temp123.txt", "a") as FH_OUT:
    print("hi", file=FH_OUT)

with open("temp123.txt","r") as FH:
    for line in FH.readlines():
        print(line)

Hello World!

hi

Quiz: Write a program to print the third line of a text file.

Show answer

#temp_file.txt should be there in the current directory
FH = open('temp_file.txt','r')
all_lines = FH.readlines()
print(all_lines[2])

This is the third sentence.

10.1 Reading files

10.2 Writing files

10.3 The with keyword

10.3 The `with` keyword