[Tut] How to Erase Contents of a File

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How to Erase Contents of a File

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Problem Formulation and Solution Overview

In this article, you’ll learn how to erase the contents of a file in Python.

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

Let’s say you have a Python script that retrieves the daily stock exchange prices for five (5) Tech Companies and saves it to

Code:

prices.txt

. To ensure no mistakes, you would like to erase the contents of this file before saving the latest data.

Question: How would we write code to erase the contents of a file?

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

• Method 1: Use
  Code:

  open()
  and
  Code:

  truncate(0)
  
  
  
• Method 2: Use
  Code:

  open()
  ,
  Code:

  seek(0)
  and
  Code:

  truncate(0)
  
  
  
• Method 3: Use
  Code:

  with open()
  
  
  
• Method 4: Use
  Code:

  open()
  and
  Code:

  close()
  on one line
  
  
• Bonus: Extract Characters and Erase Remainder
  

Preparation

To follow along, copy, paste and save the text below to

Code:

prices.txt

. Move this file to the current working directory.

AAPL,138.22
MMSF,255.67
HPE,14.51
DELL,14.51
MNDT,21.89

Method 1: Use open() and truncate()

This method erases the contents of a file without removing the file itself using

Code:

open()

and

Code:

truncate(0)

.

Code:

fp = open('prices.txt', 'w') fp.truncate(0) fp.close()

This code opens

Code:

prices.txt

in write mode (

Code:

w

) and saves the output to

Code:

fp

which creates a file object similar to the output below.

Code:

<_io.TextIOWrapper name='prices.txt' mode='w' encoding='cp1252'>

Tip: A file object is returned whenever a file is opened in Python. This object allows access to process/manipulate the open file.

Next,

Code:

fp.truncate(0)

is called. This method resizes the said file to a specified number of bytes. If no argument is passed, the current file position is used.

Finally,

Code:

fp.close()

is called to close the open file.

If this code is successful, an empty

Code:

prices.txt

file now resides in the current working directory.



Method 2: Use open(), seek(0) and truncate(0)

This method opens/re-opens a file and erases the contents without removing the file itself using

Code:

open()

,

Code:

seek()

and

Code:

truncate(0)

.

Code:

fp = open('prices.txt', 'r+') fp.seek(0) fp.truncate()

This code opens/re-opens the

Code:

prices.txt

file in read/write mode (

Code:

r+

) and saves the output to

Code:

fp

which creates a file object similar to the output above.

Next,

Code:

fp.seek(0)

is called to re-position the file pointer (

Code:

fp

) to a given position in the file. In this case, the position is (the top of the file).

Then,

Code:

fp.truncate(0)

is called. This method resizes the said file to a specified number of bytes. If no argument is passed, the current file position is used.

If this code is successful, an empty

Code:

prices.txt

file now resides in the current working directory.

Method 3: Use with open()

This method erases the contents of a file without deleting the file itself using

Code:

with open()

on one-line!

Code:

with open('prices.txt', 'w'): pass

This code calls

Code:

with open()

to open

Code:

prices.txt

for writing (

Code:

w

). Then, the

Code:

pass

statement executes, which does nothing, and the file automatically closes.

Tip: The

Code:

pass

statement is used as a placeholder. When

Code:

pass

executes, nothing happens. This is necessary when code is expected, but no code is required.

If this code is successful, an empty

Code:

prices.txt

file now resides in the current working directory.

Method 4: Use open() and close() on one line

Also a good option, this method opens a file for writing (

Code:

open()

) and closes said file (

Code:

close()

) using one line of code!

Code:

open('prices.txt', 'w').close()

This code uses

Code:

open()

to open

Code:

prices.txt

for writing (

Code:

w

). Since no other code is called, the file contents are erased, and the file closes (

Code:

close()

).

If this code is successful, an empty

Code:

prices.txt

file now resides in the current working directory.

Bonus: Erase File Contents after Specified Location

What happens if you want to erase everything after the first x number of characters in a file and return the same?

Code:

import os fp = os.open('prices.txt', os.O_RDWR|os.O_CREAT) os.ftruncate(fp, 4) os.lseek(fp, 0, 0) str = os.read(fp, 100).decode('utf-8') print(f"Read String is : {str}") os.close(fp)

This example could be used for erasing the entire contents of a file. However, let’s retrieve the first four (4) characters from

Code:

prices.txt

(AAPL) and erase the remainder.

First, this code calls in the

Code:

os

library to access the many functions available for interacting with the operating system.

Then,

Code:

prices.txt

is opened in read/write mode, and if the file does not exist, or fails, a new file is created (

Code:

os.O_RDWR|os.O_CREAT)

).

Then, the file is truncated to 4 bytes/characters (

Code:

os.ftruncate(fp, 4)

) and the file pointer (

Code:

fp

) moves to the top of the file (

Code:

os.lseek(fp, 0, 0)

).

Next, the code reads in the first four (4) bytes/characters indicated above and decodes the output (

Code:

os.read(fp, 100).decode('utf-8')

) and saves to

Code:

str

.

The output is sent to the terminal, and the file closes.

Output

Code:

Read String is: AAPL

Summary

These four (4) methods of how to erase the contents of a file should give you enough information to select the best one for your coding requirements.

Good Luck & Happy Coding!

Programmer Humor

There are only 10 kinds of people in this world: those who know binary and those who don’t.

~~~

There are 10 types of people in the world. Those who understand trinary, those who don’t, and those who mistake it for binary.