The Python pass operator is used to create empty code blocks and empty functions.
Examples of Python pass operators
Let’s see some examples using pass.
1. pass statement in code block
Let’s say we need to write a function to remove all even numbers from a list. In this case, we’ll use a for loop to traverse the numbers in the list.
If the number is divisible by 2 , then we do nothing. Otherwise, we add it to the temporary list. Finally, return a temporary list containing only odd numbers to the caller.
Python does not support empty code blocks. So we can use the pass statement here to omit the operation in the if-condition block.
def remove_evens (list_numbers): list_odds = [] for i in list_numbers: if i% 2 == 0: pass else: list_odds.append (i) return list_odds l_numbers = [1, 2, 3, 4, 5, 6] l_odds = remove_evens (l_numbers) print (l_odds)
Output: [1, 3, 5]
We don’t need any operations in the if-condition block here. So, we used the pass statement to do nothing.
2. pass statement for empty function
There is no concept of abstract functions in Python. If we need to define an empty function, we cannot write it like this.
def foo (): # TODO - implement later
Exit: IndentationError: Indented block expected
We can use the pass statement to define an empty function. The function will have an instruction, but it will not do anything.
def foo (): pass
multiple statements?
Yes, we can have multiple walk statements in a function or block of code. This is because the pass statement does not complete the function. His only work &provide an empty statement.
def bar (): pass print (’bar’) pass if True: pass pass print (’ True’) else : print (’False’) pass pass
Why do I need it?
- Python’s pass operator is very useful when defining an empty function or empty block of code.
- The most important use of the pass operator &create a contract for the classes and functions that we want to implement later. For example, we can define a Python module like this:
class EmployeeDAO: def get_emp_by_id (self, i): "" "TODO : implement this function later on: param i: employee id: return: employee object "" "pass def delete_emp (self, i): pass # This function will read Employees CSV Data file and return list of Employees def read_csv_file (file): pass
We can start implementing. Third-party code knows the functions and methods that we will be implementing, so they can continue to implement them.
Python pass statement __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.
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).
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 a file or folder in Python?
5 answers
How do I delete a file or folder in Python?
Answer #1
os.remove()removes a file.os.rmdir()removes an empty directory.shutil.rmtree()deletes a directory and all its contents.
Path objects from the Python 3.4+ pathlib module also expose these instance methods:
pathlib.Path.unlink()removes a file or symbolic link.pathlib.Path.rmdir()removes an empty directory.
Python pass statement __delete__: Questions
Answer #2
os.remove()removes a file.os.rmdir()removes an empty directory.shutil.rmtree()deletes a directory and all its contents.
Path objects from the Python 3.4+ pathlib module also expose these instance methods:
pathlib.Path.unlink()removes a file or symbolic link.pathlib.Path.rmdir()removes an empty directory.
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
- 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. Useexception 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?
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]
