Python - How To Writelines in Python

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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 

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