[Tut] How to Delete a Line from a File in Python?

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How to Delete a Line from a File in Python?

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Problem Formulation and Solution Overview

This article will show you how to delete a line from a file in Python.

To make it more interesting, we have the following running scenario:

Rivers Clothing has a flat text file,

Code:

rivers_emps.txt

containing employee data. What happens if an employee leaves? They would like you to write code to resolve this issue.

Contents of rivers_emps.txt

Code:

100:Jane Smith101:Daniel Williams102:Steve Markham103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

Question: How would we write code to remove this line?

We can accomplish this task by one of the following options:

• Method 1: Use https://blog.finxter.com/list-comprehension/List Comprehension
  
  
• Method 2: Use List Comprehension and Slicing
  
  
• Method 3: Use List Comprehension, Slicing and
  Code:

  np.savetxt()
  ,
  
  
• Method 4: Use
  Code:

  pop()
  
  
  
• Method 5: Use
  Code:

  remove()
  
  
• Method 6: Remove row(s) from a DataFrame
  

Method 1: Use List Comprehension

This example uses https://blog.finxter.com/list-comprehension/List Comprehension to remove a specific line from a flat text file.

Code:

orig_lines = [line.strip() for line in open('rivers_emps.txt')] new_lines = [l for l in orig_lines if not l.startswith('102')] with open('rivers_01.txt', 'w') as fp: print(*new_lines, sep='\n', file=fp)

The above code uses List Comprehension to read in the contents of a flat text file to a List,

Code:

orig_lines

. If output to the terminal, the following displays.

Code:

['100:Jane Smith', '101:Daniel Williams', '102:Steve Markham', '103:Howie Manson', '104:Wendy Wilson', '105:Anne McEvans', '106:Bev Doyle', '107:Hal Holden', '108:Mich Matthews', '109:Paul Paulson']

Then, List Comprehension is used again to append each element to a new List only if the element does not start with 102. If output to the terminal, the following displays.

Code:

['100:Jane Smith', '101:Daniel Williams', '103:Howie Manson', '104:Wendy Wilson', '105:Anne McEvans', '106:Bev Doyle', '107:Hal Holden', '108:Mich Matthews', '109:Paul Paulson']

As you can see, the element starting with 102 has been removed.

Next, a new file,

Code:

rivers_01.txt

, is opened in write (

Code:

w

) mode and the List created above is written to the file with a newline (

Code:

\n

) character appended to each line. The contents of the file are shown below.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

Method 2: Use List Comprehension and Slicing

This example uses List Comprehension and Slicing to remove a specific line from a flat text file.

Code:

orig_lines = [line.strip() for line in open('rivers_emps.txt')] new_lines = orig_lines[0:2] + orig_lines[3:] with open('rivers_02.txt', 'w') as fp: fp.write('\n'.join(new_lines))

The above code uses List Comprehension to read in the contents of a flat text file to a List,

Code:

orig_lines

. If output to the terminal, the following displays.

Code:

['100:Jane Smith', '101:Daniel Williams', '102:Steve Markham', '103:Howie Manson', '104:Wendy Wilson', '105:Anne McEvans', '106:Bev Doyle', '107:Hal Holden', '108:Mich Matthews', '109:Paul Paulson']

Then Slicing is used to extract all elements, except element two (2). The results save to

Code:

new_lines

. If output to the terminal, the following displays.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

As you can see, element two (2) has been removed.

Next, a new file,

Code:

rivers_02.txt

, is opened in write (

Code:

w

) mode and the List created above is written to the file with a newline (

Code:

\n

) character appended to each line. The contents of the file are shown below.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

Method 3: Use Slicing and np.savetxt()

This example uses List Comprehension, Slicing and NumPy’s

Code:

np.savetxt()

function to remove a specific line from a flat text file.

Before moving forward, please ensure that the NumPy library is installed to ensure this code runs error-free.

Code:

import numpy as np orig_lines = [line.strip() for line in open('rivers_emps.txt')] new_lines = orig_lines[0:2] + orig_lines[3:] np.savetxt('rivers_03.txt', new_lines, delimiter='\n', fmt='%s')

The first line imports the https://blog.finxter.com/numpy-tutorial/NumPy library.

The following line uses List Comprehension to read the contents of a flat text file to the List,

Code:

orig_lines

. If output to the terminal, the following displays.

Code:

['100:Jane Smith', '101:Daniel Williams', '102:Steve Markham', '103:Howie Manson', '104:Wendy Wilson', '105:Anne McEvans', '106:Bev Doyle', '107:Hal Holden', '108:Mich Matthews', '109:Paul Paulson']

Then Slicing is applied to extract all elements, except element two (2). The results save to

Code:

new_lines

. If output to the terminal, the following displays.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

As you can see, element two (2) has been removed.

The last code line calls

Code:

np.savetxt()

and passes it three (3) arguments:

• The filename (‘
  Code:

  rivers_03.txt
  ‘).
  
  
• An iterable, in this case, a List (
  Code:

  new_lines
  ).
  
  
• A delimiter (appended to each line) – a newline character (
  Code:

  \n
  ).
  
  
• The format. Strings are defined as
  Code:

  %s
  .
  

The contents of

Code:

rivers_03.txt

displays below.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

Method 4: Use pop()

This example uses the

Code:

pop()

function to remove a specific line from a flat text file.

Code:

import numpy as np orig_lines = [line.strip() for line in open('rivers_emps.txt')] orig_lines.pop(2) np.savetxt('rivers_04.txt', orig_lines, delimiter='\n', fmt='%s')

The first line imports the NumPy library.

The following line uses List Comprehension to read in the contents of a flat text file to the List,

Code:

orig_lines

. If output to the terminal, the following displays.

Code:

['100:Jane Smith', '101:Daniel Williams', '102:Steve Markham', '103:Howie Manson', '104:Wendy Wilson', '105:Anne McEvans', '106:Bev Doyle', '107:Hal Holden', '108:Mich Matthews', '109:Paul Paulson']

Then, the

Code:

pop()

method is called and passed one (1) argument, the element’s index to remove.

In this case, it is the second element.

If this List was output to the terminal, the following would display.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

As shown in Method 3, the results save to a flat text file. In this case,

Code:

rivers_04.txt

. The contents are the same as in the previous examples.



Note: The

Code:

pop()

function removes the appropriate index and returns the contents to capture if necessary.

Method 5: Use remove()

This example uses the

Code:

remove()

function to remove a specific line from a flat text file.

Code:

import numpy as np orig_lines = [line.strip() for line in open('rivers_emps.txt')] orig_lines.remove('102:Steve Markham') np.savetxt('rivers_05.txt', orig_lines, delimiter='\n', fmt='%s')

This code works exactly like the code in Method 4. However, instead of passing a location of the element to remove, this function requires the contents of the entire line you to remove.

Then, the

Code:

remove()

function is called and passed one (1) argument, the index to remove. In this case, it is the second element. If this List was output to the terminal, the following would display.

Code:

100:Jane Smith101:Daniel Williams103:Howie Manson104:Wendy Wilson105:Anne McEvans106:Bev Doyle107:Hal Holden108:Mich Matthews109:Paul Paulson

As shown in the previous examples, the results save to a flat text file. In this case,

Code:

rivers_05.txt

.



Bonus: Remove row(s) from a DataFrame

CSV files are also known as flat-text files. This code shows you how to easily remove single or multiple rows from a CSV file

Code:

import pandas as pd import numpy as np staff = { 'First' : ['Alice', 'Micah', 'James', 'Mark'], 'Last' : ['Smith', 'Jones', 'Watts', 'Hunter'], 'Rate' : [30, 40, 50, 37], 'Age' : [23, 29, 19, 45]} indexes=['FName', 'LName', 'Rate', 'Age'] df = pd.DataFrame(staff, index=indexes) df1 = df.drop(index=['Age']) df.to_csv('staff.csv', index=False)

Finxter Challenge
Find 2 Additional Ways to Remove Lines

Summary

This article has provided five (5) ways to delete a line from a file to select the best fit for your coding requirements.

Good Luck & Happy Coding!

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