How to extract specific portions of a text file using Python

Extracting text from a file is a common task in scripting and programming, and Python makes it easy. In this guide, we'll discuss some simple ways to extract text from a file using the Python 3 programming language.

Make sure you're using Python 3

In this guide, we'll be using Python version 3. Most systems come pre-installed with Python 2.7. While Python 2.7 is used in legacy code, Python 3 is the present and future of the Python language. Unless you have a specific reason to write or support Python 2, we recommend working in Python 3.

For Microsoft Windows, Python 3 can be downloaded from the Python official website. When installing, make sure the "Install launcher for all users" and "Add Python to PATH" options are both checked, as shown in the image below.

On Linux, you can install Python 3 with your package manager. For instance, on Debian or Ubuntu, you can install it with the following command:

sudo apt-get update && sudo apt-get install python3

For macOS, the Python 3 installer can be downloaded from python.org, as linked above. If you are using the Homebrew package manager, it can also be installed by opening a terminal window (Applications → Utilities), and running this command:

brew install python3

Running Python

On Linux and macOS, the command to run the Python 3 interpreter is python3. On Windows, if you installed the launcher, the command is py. The commands on this page use python3; if you're on Windows, substitute py for python3 in all commands.

Running Python with no options starts the interactive interpreter. For more information about using the interpreter, see Python overview: using the Python interpreter. If you accidentally enter the interpreter, you can exit it using the command exit() or quit().

Running Python with a file name will interpret that python program. For instance:

python3 program.py

...runs the program contained in the file program.py.

Okay, how can we use Python to extract text from a text file?

Reading data from a text file

First, let's read a text file. Let's say we're working with a file named lorem.txt, which contains lines from the Lorem Ipsum example text.

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nunc fringilla arcu congue metus aliquam mollis.
Mauris nec maximus purus. Maecenas sit amet pretium tellus.
Quisque at dignissim lacus.

A Python program can read a text file using the built-in open() function. For example, the Python 3 program below opens lorem.txt for reading in text mode, reads the contents into a string variable named contents, closes the file, and prints the data.

myfile = open("lorem.txt", "rt") # open lorem.txt for reading text
contents = myfile.read()         # read the entire file to string
myfile.close()                   # close the file
print(contents)                  # print string contents

Here, myfile is the name we give to our file object.

The "rt" parameter in the open() function means "we're opening this file to read text data"

The hash mark ("#") means that everything on that line is a comment, and it's ignored by the Python interpreter.

If you save this program in a file called read.py, you can run it with the following command.

python3 read.py

The command above outputs the contents of lorem.txt:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nunc fringilla arcu congue metus aliquam mollis.
Mauris nec maximus purus. Maecenas sit amet pretium tellus.
Quisque at dignissim lacus.

Using "with open"

It's important to close your open files as soon as possible: open the file, perform your operation, and close it. Don't leave it open for extended periods of time.

When you're working with files, it's good practice to use the with open...as compound statement. It's the cleanest way to open a file, operate on it, and close the file, all in one easy-to-read block of code. The file is automatically closed when the code block completes.

Using with open...as, we can rewrite our program to look like this:

with open ('lorem.txt', 'rt') as myfile:  # Open lorem.txt for reading text
    contents = myfile.read()              # Read the entire file to a string
print(contents)                           # Print the string

Example

Save the program as read.py and execute it:

python3 read.py

Output:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nunc fringilla arcu congue metus aliquam mollis.
Mauris nec maximus purus. Maecenas sit amet pretium tellus.
Quisque at dignissim lacus.

Reading text files line-by-line

In the examples so far, we've been reading in the whole file at once. Reading a full file is no big deal with small files, but generally, it's not a great idea. For one thing, if your file is bigger than the amount of available memory, you'll encounter an error.

In almost every case, it's a better idea to read a text file one line at a time.

In Python, the file object is an iterator. An iterator is a type of Python object which behaves in certain ways when operated on repeatedly. For instance, you can use a for loop to operate on a file object repeatedly, and each time the same operation is performed, you'll receive a different, or "next," result.

Example

For text files, the file object iterates one line of text at a time. It considers one line of text a "unit" of data, so we can use a for...in loop statement to iterate one line at a time:

with open ('lorem.txt', 'rt') as myfile:  # Open lorem.txt for reading
    for myline in myfile:              # For each line, read to a string,
        print(myline)                  # and print the string.

Output:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.

Nunc fringilla arcu congue metus aliquam mollis.

Mauris nec maximus purus. Maecenas sit amet pretium tellus.

Quisque at dignissim lacus.

Notice that we're getting an extra line break ("newline") after every line. That's because two newlines are being printed. The first one is the newline at the end of every line of our text file. The second newline happens because, by default, print() adds a linebreak of its own at the end of whatever you've asked it to print.

Let's store our lines of text in a variable — specifically, a list variable — so we can look at it more closely.

Storing text data in a variable

In Python, lists are similar to, but not the same as, an array in C or Java. A Python list contains indexed data, of varying lengths and types.

Example

mylines = []                             # Declare an empty list named mylines.
with open ('lorem.txt', 'rt') as myfile: # Open lorem.txt for reading text data.
    for myline in myfile:                # For each line, stored as myline,
        mylines.append(myline)           # add its contents to mylines.
print(mylines)                           # Print the list.

The output of this program is a little different. Instead of printing the contents of the list, this program prints our list object, which looks like this:

Output:

['Lorem ipsum dolor sit amet, consectetur adipiscing elit.\n', 'Nunc fringilla arcu congue metus aliquam mollis.\n', 'Mauris nec maximus purus. Maecenas sit amet pretium tellus.\n', 'Quisque at dignissim lacus.\n']

Here, we see the raw contents of the list. In its raw object form, a list is represented as a comma-delimited list. Here, each element is represented as a string, and each newline is represented as its escape character sequence, \n.

Much like a C or Java array, the list elements are accessed by specifying an index number after the variable name, in brackets. Index numbers start at zero — other words, the nth element of a list has the numeric index n-1.

Example

We can print the first element of lines by specifying index number 0, contained in brackets after the name of the list:

print(mylines[0])

Output:

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc fringilla arcu congue metus aliquam mollis.

Example

Or the third line, by specifying index number 2:

print(mylines[2])

Output:

Quisque at dignissim lacus.

But if we try to access an index for which there is no value, we get an error:

Example

print(mylines[3])

Output:

Traceback (most recent call last):
File <filename>, line <linenum>, in <module>
print(mylines[3])
IndexError: list index out of range

Example

A list object is an iterator, so to print every element of the list, we can iterate over it with for...in:

mylines = []                              # Declare an empty list
with open ('lorem.txt', 'rt') as myfile:  # Open lorem.txt for reading text.
    for line in myfile:                   # For each line of text,
        mylines.append(line)              # add that line to the list.
    for element in mylines:               # For each element in the list,
        print(element)                    # print it.

Output:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.

Nunc fringilla arcu congue metus aliquam mollis.

Mauris nec maximus purus. Maecenas sit amet pretium tellus.

Quisque at dignissim lacus.

But we're still getting extra newlines. Each line of our text file ends in a newline character ('\n'), which is being printed. Also, after printing each line, print() adds a newline of its own, unless you tell it to do otherwise.

We can change this default behavior by specifying an end parameter in our print() call:

print(element, end='')

By setting end to an empty string (two single quotes, with no space), we tell print() to print nothing at the end of a line, instead of a newline character.

Example

Our revised program looks like this:

mylines = []                              # Declare an empty list
with open ('lorem.txt', 'rt') as myfile:  # Open file lorem.txt
    for line in myfile:                   # For each line of text,
        mylines.append(line)              # add that line to the list.
    for element in mylines:               # For each element in the list,
        print(element, end='')            # print it without extra newlines.

Output:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nunc fringilla arcu congue metus aliquam mollis.
Mauris nec maximus purus. Maecenas sit amet pretium tellus.
Quisque at dignissim lacus.

The newlines you see here are actually in the file; they're a special character ('\n') at the end of each line. We want to get rid of these, so we don't have to worry about them while we process the file.

How to strip newlines

To remove the newlines completely, we can strip them. To strip a string is to remove one or more characters, usually whitespace, from either the beginning or end of the string.

Python 3 string objects have a method called rstrip(), which strips characters from the right side of a string. The English language reads left-to-right, so stripping from the right side removes characters from the end.

If the variable is named mystring, we can strip its right side with mystring.rstrip(chars), where chars is a string of characters to strip. For example, "123abc".rstrip("bc") returns 123a.

The statement string.rstrip('\n') will strip a newline character from the right side of string. The following version of our program strips the newlines when each line is read from the text file:

mylines = []                                # Declare an empty list.
with open ('lorem.txt', 'rt') as myfile:    # Open lorem.txt for reading text.
    for myline in myfile:                   # For each line in the file,
        mylines.append(myline.rstrip('\n')) # strip newline and add to list.
for element in mylines:                     # For each element in the list,
    print(element)                          # print it.

The text is now stored in a list variable, so individual lines can be accessed by index number. Newlines were stripped, so we don't have to worry about them. We can always put them back later if we reconstruct the file and write it to disk.

Now, let's search the lines in the list for a specific substring.

Searching text for a substring

Let's say we want to locate every occurrence of a certain phrase, or even a single letter. For instance, maybe we need to know where every "e" is. We can accomplish this using the string's find() method.

The list stores each line of our text as a string object. All string objects have a method, find(), which locates the first occurrence of a substrings in the string.

Let's use the find() method to search for the letter "e" in the first line of our text file, which is stored in the list mylines. The first element of mylines is a string object containing the first line of the text file. This string object has a find() method.

In the parentheses of find(), we specify parameters. The first and only required parameter is the string to search for, "e". The statement mylines[0].find("e") tells the interpreter to search forward, starting at the beginning of the string, one character at a time, until it finds the letter "e." When it finds one, it stops searching, and returns the index number where that "e" is located. If it reaches the end of the string, it returns -1 to indicate nothing was found.

Example

print(mylines[0].find("e"))

Output:

3

The return value "3" tells us that the letter "e" is the fourth character, the "e" in "Lorem". (Remember, the index is zero-based: index 0 is the first character, 1 is the second, etc.)

The find() method takes two optional, additional parameters: a start index and a stop index, indicating where in the string the search should begin and end. For instance, string.find("abc", 10, 20) searches for the substring "abc", but only from the 11th to the 21st character. If stop is not specified, find() starts at index start, and stops at the end of the string.

Example

For instance, the following statement searchs for "e" in mylines[0], beginning at the fifth character.

print(mylines[0].find("e", 4))

Output:

24

In other words, starting at the 5th character in line[0], the first "e" is located at index 24 (the "e" in "nec").

Example

To start searching at index 10, and stop at index 30:

print(mylines[1].find("e", 10, 30))

Output:

28

(The first "e" in "Maecenas").

If find() doesn't locate the substring in the search range, it returns the number -1, indicating failure:

print(mylines[0].find("e", 25, 30))

Output:

-1

There were no "e" occurrences between indices 25 and 30.

Finding all occurrences of a substring

But what if we want to locate every occurrence of a substring, not just the first one we encounter? We can iterate over the string, starting from the index of the previous match.

In this example, we'll use a while loop to repeatedly find the letter "e". When an occurrence is found, we call find again, starting from a new location in the string. Specifically, the location of the last occurrence, plus the length of the string (so we can move forward past the last one). When find returns -1, or the start index exceeds the length of the string, we stop.

# Build array of lines from file, strip newlines

mylines = []                                # Declare an empty list.
with open ('lorem.txt', 'rt') as myfile:    # Open lorem.txt for reading text.
    for myline in myfile:                   # For each line in the file,
        mylines.append(myline.rstrip('\n')) # strip newline and add to list.

# Locate and print all occurences of letter "e"

substr = "e"                  # substring to search for.
for line in mylines:          # string to be searched
  index = 0                   # current index: character being compared
  prev = 0                    # previous index: last character compared
  while index < len(line):    # While index has not exceeded string length,
    index = line.find(substr, index)  # set index to first occurrence of "e"
    if index == -1:           # If nothing was found,
      break                   # exit the while loop.
    print(" " * (index - prev) + "e", end='')  # print spaces from previous
                                               # match, then the substring.
    prev = index + len(substr)       # remember this position for next loop.
    index += len(substr)      # increment the index by the length of substr.
                              # (Repeat until index > line length)
  print('\n' + line);         # Print the original string under the e's

Output:

   e                    e       e  e               e
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
                         e  e
Nunc fringilla arcu congue metus aliquam mollis.
        e                   e e          e    e      e
Mauris nec maximus purus. Maecenas sit amet pretium tellus.
      e
Quisque at dignissim lacus.

Incorporating regular expressions

For complex searches, use regular expressions.

The Python regular expressions module is called re. To use it in your program, import the module before you use it:

import re

The re module implements regular expressions by compiling a search pattern into a pattern object. Methods of this object can then be used to perform match operations.

For example, let's say you want to search for any word in your document which starts with the letter d and ends in the letter r. We can accomplish this using the regular expression "\bd\w*r\b". What does this mean?

So this regular expression will match any string that can be described as "a word boundary, then a lowercase 'd', then zero or more word characters, then a lowercase 'r', then a word boundary." Strings described this way include the words destroyer, dour, and doctor, and the abbreviation dr.

To use this regular expression in Python search operations, we first compile it into a pattern object. For instance, the following Python statement creates a pattern object named pattern which we can use to perform searches using that regular expression.

pattern = re.compile(r"\bd\w*r\b")

Now we can use the pattern object's methods, such as search(), to search a string for the compiled regular expression, looking for a match. If it finds one, it returns a special result called a match object. Otherwise, it returns None, a built-in Python constant used like the boolean value "false".

import re
str = "Good morning, doctor."
pat = re.compile(r"\bd\w*r\b")  # compile regex "\bd\w*r\b" to a pattern object
if pat.search(str) != None:     # Search for the pattern. If found,
    print("Found it.")

Output:

Found it.

To perform a case-insensitive search, you can specify the special constant re.IGNORECASE in the compile step:

import re
str = "Hello, DoctoR."
pat = re.compile(r"\bd\w*r\b", re.IGNORECASE)  # upper and lowercase will match
if pat.search(str) != None:
    print("Found it.")

Output:

Found it.

Putting it all together

So now we know how to open a file, read the lines into a list, and locate a substring in any given list element. Let's use this knowledge to build some example programs.

Print all lines containing substring

The program below reads a log file line by line. If the line contains the word "error," it is added to a list called errors. If not, it is ignored. The lower() string method converts all strings to lowercase for comparison purposes, making the search case-insensitive without altering the original strings.

Note that the find() method is called directly on the result of the lower() method; this is called method chaining. Also, note that in the print() statement, we construct an output string by joining several strings with the + operator.

errors = []                       # The list where we will store results.
linenum = 0
substr = "error".lower()          # Substring to search for.
with open ('logfile.txt', 'rt') as myfile:
    for line in myfile:
        linenum += 1
        if line.lower().find(substr) != -1:    # if case-insensitive match,
            errors.append("Line " + str(linenum) + ": " + line.rstrip('\n'))
for err in errors:
    print(err)

Input (stored in logfile.txt):

This is line 1
This is line 2
Line 3 has an error!
This is line 4
Line 5 also has an error!

Output:

Line 3: Line 3 has an error!
Line 5: Line 5 also has an error!

Extract all lines containing substring, using regex

The program below is similar to the above program, but using the re regular expressions module. The errors and line numbers are stored as tuples, e.g., (linenum, line). The tuple is created by the additional enclosing parentheses in the errors.append() statement. The elements of the tuple are referenced similar to a list, with a zero-based index in brackets. As constructed here, err[0] is a linenum and err[1] is the associated line containing an error.

import re
errors = []
linenum = 0
pattern = re.compile("error", re.IGNORECASE)  # Compile a case-insensitive regex
with open ('logfile.txt', 'rt') as myfile:    
    for line in myfile:
        linenum += 1
        if pattern.search(line) != None:      # If a match is found 
            errors.append((linenum, line.rstrip('\n')))
for err in errors:                            # Iterate over the list of tuples
    print("Line " + str(err[0]) + ": " + err[1])

Output:

Line 6: Mar 28 09:10:37 Error: cannot contact server. Connection refused.
Line 10: Mar 28 10:28:15 Kernel error: The specified location is not mounted.
Line 14: Mar 28 11:06:30 ERROR: usb 1-1: can't set config, exiting.

Extract all lines containing a phone number

The program below prints any line of a text file, info.txt, which contains a US or international phone number. It accomplishes this with the regular expression "(\+\d{1,2})?[\s.-]?\d{3}[\s.-]?\d{4}". This regex matches the following phone number notations:

• 123-456-7890
• (123) 456-7890
• 123 456 7890
• 123.456.7890
• +91 (123) 456-7890

import re
errors = []
linenum = 0
pattern = re.compile(r"(\+\d{1,2})?[\s.-]?\d{3}[\s.-]?\d{4}")
with open ('info.txt', 'rt') as myfile:
    for line in myfile:
        linenum += 1
        if pattern.search(line) != None:  # If pattern search finds a match,
            errors.append((linenum, line.rstrip('\n')))
for err in errors:
    print("Line ", str(err[0]), ": " + err[1])

Output:

Line  3 : My phone number is 731.215.8881.
Line  7 : You can reach Mr. Walters at (212) 558-3131.
Line  12 : His agent, Mrs. Kennedy, can be reached at +12 (123) 456-7890
Line  14 : She can also be contacted at (888) 312.8403, extension 12.

Search a dictionary for words

The program below searches the dictionary for any words that start with h and end in pe. For input, it uses a dictionary file included on many Unix systems, /usr/share/dict/words.

import re
filename = "/usr/share/dict/words"
pattern = re.compile(r"\bh\w*pe$", re.IGNORECASE)
with open(filename, "rt") as myfile:
    for line in myfile:
        if pattern.search(line) != None:
            print(line, end='')

Output:

Hope
heliotrope
hope
hornpipe
horoscope
hype