Python - How To Writelines in Python

93

This tutorial will introduce and explain the difference between the write() and writelines() methods in Python.

Use the write() and writelines() Methods to Write Specified Text to a File in Python

The write() method expects a string as an argument and writes it to the file. If we provide a list of strings, it will raise an exception. The writelines() method expects an iterable argument. Also, the write() method displays the output but does not provide a new line character, whereas the writelines() method displays the output and provides a new line character at the end of the string. It is important to note that writelines() do not automatically insert a new line after each item in the iterable. We have to provide a new line by ourselves. If we have many lines to write to a file, writelines() could be a better option. It performs well because it doesn’t create a temporary concatenated string, just iterating over the lines.

In the program below, we opened the file samplefile.txt in the write mode. The lines variable is a tuple, i.e., an immutable collection of strings. When we try to execute the program, it raised an exception-type error. The arguments we provided to the write() method is a tuple that caused the error. Here, we used the with method for file handling. This method provides better syntax and exception handling. So, it is a good practice to use the with method where applicable. The method is helpful because any files opened will be closed after one is done.

Example Code:

# python 3.x nl = "\n" line1 = "Good" line2 = "Morning" line3 = "Sunshine" lines = line1, nl, line2, nl, line3, nl with open('samplefile.txt', 'w') as f:     f.write(lines) 

Output:

TypeError: write() argument must be str, not tuple 

In a similar context, when we use writelines(), the program is executed successfully, and a file named samplefile.txt is created where lines is passed as an argument to the writelines() method.

Example Code:

# python 3.x nl = "\n" line1 = "Good" line2 = "Morning" line3 = "Sunshine" lines = line1, nl, line2, nl, line3, nl with open('samplefile.txt', 'w') as f:     f.writelines(lines) 

Output:

Good Morning Sunshine 

For the same program, the multiple lines are concatenated to a single string variable called lines_joined, which is easily accepted as an argument for the write() method.

Example Code:

# python 3.x nl = "\n" line1 = "Good" line2 = "Morning" line3 = "Sunshine"  lines_joined = line1 + nl + line2 + nl + line3 + nl with open('samplefile.txt', 'w') as f:     f.write(lines_joined) 

Output:

Good Morning Sunshine 

Use the write() and writelines() Methods to Write Iterables to a File in Python

The join() method takes all items in an iterable and joins them into one string. In the program below, we used the join() method with '\n' as a glue to concatenate the string in lines. It is a better approach for concatenation instead of using the + operator.

Example Code:

# python 3.x lines = ["Hello", "World"] with open('hello.txt', 'w') as f:     f.write('\n'.join(lines)) 

Output:

Hello World 

It is less efficient to use the join() method for an extremely long list of strings. In such a case, an entirely new and very long string is created in memory before writing it. So, in the program below, we used a generator expression to write the strings in the newline. Here, the data is written piece-wise.

Example Code:

# python 3.x lines = ["Hello", "World"] with open('hello.txt', 'w') as f:     for l in lines:         f.write('%s\n' % l) 

Output:

Hello World 

The same program as above is implemented through the writelines() method. It makes the use of a generator expression and dynamically creates newline-terminated strings. The writelines() method iterates over this sequence of strings and writes every item.

Example Code:

# python 3.x  lines = ["Hello", "World"] with open('hello.txt', 'w') as file:     file.writelines("%s\n" % t for t in texts) 

Output:

Hello World