How to Read Lines in a File and Do Something With It in Python

Opening a file and reading the content of a file is 1 of the common things you would practice while doing data analysis.

In this tutorial, we will come across 3 examples of reading a text file in Python iii.

One easy way to read a text file and parse each line is to use the python statement "readlines" on a file object.

How To Read all lines in a file at in one case? Use readlines()

If you want to read all lines of a file at the same time, Python's readlines() office is for you. Python's readlines role reads everything in the text file and has them in a listing of lines. Here is an case of how to utilize Python'southward readlines.

We first open up the file using open() function in read only mode. And utilize the file handler from opening the file to read all lines using readlines() every bit follows.

# Open the file with read only allow f = open('my_text_file.txt', "r") # apply readlines to read all lines in the file # The variable "lines" is a list containing all lines in the file lines = f.readlines() # close the file after reading the lines. f.close()          

We can also read all the lines of a file at once in another style. Basically, we would use the file handler object after opening the file as argument to listing() function to get all the lines every bit a list.

Another way to read lines at once is to just use

# read all lines at once lines = list(f)          

Note that the terminal grapheme of each line is newline character.

Then you can go over the list of "lines" to parse each line. As you tin can immediately find, "readlines" or "list(f) works great for a small text file. However, it is not memory efficient to use if your text files are really large. A better way to read a text file that is memory-friendly is to read the file line past line, that is one line at a time.

Cadre Python has (at least) 2 ways to read a text file line past line hands.

How To Read a Text File Line past Line Using While Argument in Python?

Hither is the way to read text file one line at a time using "While" statement and python's readline function. Since we read 1 line at a fourth dimension with readline, we tin easily handle large files without worrying nearly memory problems.

# Open the file with read but permit f = open('my_text_file.txt') # apply readline() to read the first line  line = f.readline() # use the read line to read further. # If the file is not empty proceed reading one line # at a time, till the file is empty while line:     # in python two+     # print line     # in python 3 print is a builtin role, and then     print(line)     # utilise realine() to read next line     line = f.readline() f.shut()          

Another variation of reading a file with while statement and readline statement is every bit follows. Here the while tests for boolean and read line by line until we reach the terminate of file and line will be empty.

# file handle fh fh = open('my_text_file.txt') while True:     # read line     line = fh.readline()     # in python 2, print line     # in python 3     impress(line)     # bank check if line is non empty     if not line:         intermission fh.shut()          

How To Read a Text File Line by Line Using an Iterator in Python?

One tin can also use an iterator to read a text file one line at time. Here is how to do it.

fh = open('my_text_file.txt') for line in fh:     # in python 2     # print line     # in python 3     print(line) fh.close()          

Remembering to close the file handler ("fh") with the statement "fh.close()" tin exist hard initially. One can check if a file handler is closed with

# bank check if the file file handler is airtight or not >fh.closed # true if the file handler is airtight  True          

One tin open files in a much simpler manner using "with" argument in Python, without having to close the file handler. The with operator creates a context manager and it volition automatically shut the file for you when you are done with it.

Check here to see how to use "with" argument to open file.

• "with" statement in Python to Open up a file

Do yous want to read a text file line by line and skip comment lines?, check this post

• three Ways to Read a File and Skip Initial Annotate lines in Python

Exercise you lot want to read/load text file numerical information, cheque this postal service

• How to read a numerical data/file in Python with numpy?

Do you accept data in a csv or tab limited text file and want to read it in Python? The best option is to use Python's pandas package. Here is how to load information files in python with Pandas,

• vii Tips to Read a CSV File as Pandas Information Frame

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Source: https://cmdlinetips.com/2011/08/three-ways-to-read-a-text-file-line-by-line-in-python/

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