How to delete an item in a list if it exists?

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I am getting new_tag from a form text field with self.response.get("new_tag") and selected_tags from checkbox fields with

self.response.get_all("selected_tags")

I combine them like this:

tag_string = new_tag
new_tag_list = f1.striplist(tag_string.split(";") + selected_tags)

(f1.striplist is a function that strips white spaces inside the strings in the list.)

But in the case that tag_list is empty (no new tags are entered) but there are some selected_tags, new_tag_list contains an empty string " ".

For example, from logging.info:

new_tag
selected_tags[u"Hello", u"Cool", u"Glam"]
new_tag_list[u"", u"Hello", u"Cool", u"Glam"]

How do I get rid of the empty string?

If there is an empty string in the list:

>>> s = [u"", u"Hello", u"Cool", u"Glam"]
>>> i = s.index("")
>>> del s[i]
>>> s
[u"Hello", u"Cool", u"Glam"]

But if there is no empty string:

>>> s = [u"Hello", u"Cool", u"Glam"]
>>> if s.index(""):
        i = s.index("")
        del s[i]
    else:
        print "new_tag_list has no empty string"

But this gives:

Traceback (most recent call last):
  File "<pyshell#30>", line 1, in <module>
    if new_tag_list.index(""):
        ValueError: list.index(x): x not in list

Why does this happen, and how do I work around it?

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How to delete an item in a list if it exists? __del__: Questions

How can I make a time delay in Python?

5 answers

I would like to know how to put a time delay in a Python script.

2973

Answer #1

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

Here is another example where something is run approximately once a minute:

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

2973

Answer #2

You can use the sleep() function in the time module. It can take a float argument for sub-second resolution.

from time import sleep
sleep(0.1) # Time in seconds

How to delete an item in a list if it exists? __del__: Questions

How to delete a file or folder in Python?

5 answers

How do I delete a file or folder in Python?

2639

Answer #1


Path objects from the Python 3.4+ pathlib module also expose these instance methods:

2639

Answer #2


Path objects from the Python 3.4+ pathlib module also expose these instance methods:

2639

Answer #3

Python syntax to delete a file

import os
os.remove("/tmp/<file_name>.txt")

Or

import os
os.unlink("/tmp/<file_name>.txt")

Or

pathlib Library for Python version >= 3.4

file_to_rem = pathlib.Path("/tmp/<file_name>.txt")
file_to_rem.unlink()

Path.unlink(missing_ok=False)

Unlink method used to remove the file or the symbolik link.

If missing_ok is false (the default), FileNotFoundError is raised if the path does not exist.
If missing_ok is true, FileNotFoundError exceptions will be ignored (same behavior as the POSIX rm -f command).
Changed in version 3.8: The missing_ok parameter was added.

Best practice

  1. First, check whether the file or folder exists or not then only delete that file. This can be achieved in two ways :
    a. os.path.isfile("/path/to/file")
    b. Use exception handling.

EXAMPLE for os.path.isfile

#!/usr/bin/python
import os
myfile="/tmp/foo.txt"

## If file exists, delete it ##
if os.path.isfile(myfile):
    os.remove(myfile)
else:    ## Show an error ##
    print("Error: %s file not found" % myfile)

Exception Handling

#!/usr/bin/python
import os

## Get input ##
myfile= raw_input("Enter file name to delete: ")

## Try to delete the file ##
try:
    os.remove(myfile)
except OSError as e:  ## if failed, report it back to the user ##
    print ("Error: %s - %s." % (e.filename, e.strerror))

RESPECTIVE OUTPUT

Enter file name to delete : demo.txt
Error: demo.txt - No such file or directory.

Enter file name to delete : rrr.txt
Error: rrr.txt - Operation not permitted.

Enter file name to delete : foo.txt

Python syntax to delete a folder

shutil.rmtree()

Example for shutil.rmtree()

#!/usr/bin/python
import os
import sys
import shutil

# Get directory name
mydir= raw_input("Enter directory name: ")

## Try to remove tree; if failed show an error using try...except on screen
try:
    shutil.rmtree(mydir)
except OSError as e:
    print ("Error: %s - %s." % (e.filename, e.strerror))

Is there a simple way to delete a list element by value?

5 answers

I want to remove a value from a list if it exists in the list (which it may not).

a = [1, 2, 3, 4]
b = a.index(6)

del a[b]
print(a)

The above case (in which it does not exist) shows the following error:

Traceback (most recent call last):
  File "D:zjm_codea.py", line 6, in <module>
    b = a.index(6)
ValueError: list.index(x): x not in list

So I have to do this:

a = [1, 2, 3, 4]

try:
    b = a.index(6)
    del a[b]
except:
    pass

print(a)

But is there not a simpler way to do this?

1055

Answer #1

To remove an element"s first occurrence in a list, simply use list.remove:

>>> a = ["a", "b", "c", "d"]
>>> a.remove("b")
>>> print(a)
["a", "c", "d"]

Mind that it does not remove all occurrences of your element. Use a list comprehension for that.

>>> a = [10, 20, 30, 40, 20, 30, 40, 20, 70, 20]
>>> a = [x for x in a if x != 20]
>>> print(a)
[10, 30, 40, 30, 40, 70]

We hope this article has helped you to resolve the problem. Apart from How to delete an item in a list if it exists?, check other __del__-related topics.

Want to excel in Python? See our review of the best Python online courses 2022. If you are interested in Data Science, check also how to learn programming in R.

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Frank Gonzalez

Milan | 2022-11-30

__delete__ is always a bit confusing 😭 How to delete an item in a list if it exists? is not the only problem I encountered. Will get back tomorrow with feedback

Xu Schteiner

Shanghai | 2022-11-30

split is always a bit confusing 😭 How to delete an item in a list if it exists? is not the only problem I encountered. I am just not quite sure it is the best method

Cornwall Sikorski

Rome | 2022-11-30

Simply put and clear. Thank you for sharing. How to delete an item in a list if it exists? and other issues with StackOverflow was always my weak point 😁. I am just not quite sure it is the best method

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