Descriptor in Python

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Descriptor protocol:
In other programming languages, descriptors are called setter and getter, where public functions are used to get and set a private variable. Python has no concept of private variables, and the descriptor protocol can be thought of as a Pythonic way to achieve something like this. Descriptors — it is a new way of implementing classes in Python, and it doesn’t need to inherit anything from a particular object. To easily implement descriptors in python, we must use at least one of the methods defined above. Note that the instance below returns the object that was accessed by the attribute and that the owner is the class in which the handle was assigned as an attribute. There are three protocols in the Python descriptor for setters, getters, and a delete method.

  • gfg .__ get __ (self, obj, type = None) : This attribute is called when you want to get the information (value = obj.attr) and whatever it returns is — this is what will be passed to the code that requested the attribute value.
  • gfg .__ set __ (self, obj, value) : This method is called to set the attribute values ‚Äã‚Äã (obj .attr = & # 39; value & # 39;) and it won’t return anything to you.
  • gfg .__ delete __ (self, obj) : This method is called when an attribute is removed from an object (del obj.attr)

Descriptor invocation:
Descriptors are invoked automatically when a invocation is received for the set () method or the get () method. For example, obj.gfg looks for gfg in the obj dictionary. If gfg defines a __get __ () method, then gfg .__ get __ (obj) . It can also be called directly by the method name, i.e. gfg .__ get __ (obj) .

# Show Python program
# how to call the descriptor

def __ getattribute __ ( self , key):

v = object .__ getattribute __ ( self , key)

if hasattr ( v, ’__get__’ ):

return v .__ get __ ( None , self )

return v

Important points to remember:

  • Descriptors are called by the __getattribute __ () method.
  • Overriding __getattribute__ () prevents automatic calls to the descriptor.
  • object .__ getattribute __ () and type .__ getattribute __ () various calls to __get __ ( ) .
  • Data descriptors always override instance dictionaries.
  • Non-data descriptors can be overridden by instance dictionaries.

An example of a descriptor:
In this example, a data descriptor usually sets and returns values ‚Äã‚Äãand prints t a message logging their access.
Code 1:

class Descriptor ( object ):

def __ init __ ( self , name = ’ ’):

self . name = name

def __ get __ ( self , obj, objtype):

return " {} for {} " . format ( self .name, self . name)

def __ set __ ( self , obj, name):

if isinstance (name, str ):

self . name = name

else :

raise TypeError ( " Name should be string " )

class GFG ( object ):

name = Descriptor ()

g = GFG ()

g.name = "Geeks"

print (g.name)

Exit:

Python.Engineering

Code 2:

class Descriptor ( object ):

def __ init __ ( self , name = ’ ’):

self . name = name

def __ get __ ( self , obj, objtype):

return "{} for {}" . format ( self . name, self . name)

def __ set __ ( self , obj, name):

if isinstance ( name, str ):

self . name = name

else :

raise TypeError ( "Name should be string" )

class GFG ( object ):

name = Descriptor ()

g = GFG ()

g.name = "Computer"

print ( g.name)

Exit:

 ComputerforComputer 

Create a handle using property ():
property () , it’s easy to create a usable descriptor for any attribute. Syntax for creating property()

 property (fget = None, fset = None, fdel = None, doc = None) 

# Python program to explain the property () function


# Alphabet class

class Alphabet:

def __ init __ ( self , value):

self ._ value = value

# getting values ‚Äã‚Äã

def getValue ( self ):

print ( ’Getting value’ )

return self ._ value

# installation values ‚Äã‚Äã

def setValue ( self , value):

print ( ’Setting value to’ + value)

self ._ value = value

# remove values ‚Äã‚Äã

def delValue ( self ):

print ( ’Deleting value’ )

del self ._ value

value = property (getValue, setValue, delValue,)


# pass the value

x = Alphabet ( ’GeeksforGeeks’ )

print (x.value)

x.value = ’GfG’

del x.value

Exit:

 Getting value Python.Engineering Setting value to GfG Deleting value 

Generating a descriptor using class methods:
In this we created Let’s get the class and override any of the descriptor methods __set__ , __ get__ , and __delete__ . This method is used when the same descriptor is required for many different classes and attributes, for example, for type checking.

class Descriptor ( object ):

def __ init__ ( self , name = ’ ’):

self .name = name

def __ get __ ( self , obj, objtype):

return " {} for {} " . format ( self . name, self . name)

def __ set __ ( self , obj, name):

if isinstance (name, str ):

self . name = name

else :

raise TypeError ( "Name should be string" )

class GFG ( object ):

name = Descriptor ()

g = GFG ()

g.name = "Geeks"

print (g.name)

Output:

 Python.Engineering 

Create a descriptor using @property Decorator:
In this we use the power of property decorators, which are a combination of a property type method and Python decorators.

class Alphabet:

def __ init __ ( self , value):

self ._ value = value

# by retrieving values ‚Äã‚Äã

@ property

def value ( self ):

print ( ’Getting value’ )

return self ._ value

# setting values ‚Äã‚Äã

@ value . setter

def value ( self , value ):

print ( ’Setting value to ’ + value)

self ._ value = value

# remove values ‚Äã‚Äã

@ value .deleter

def value ( self ):

print ( ’Deleting value’ )

del self ._ value


# pass the value

x = Alphabet ( ’Peter’ )

print (x.value)

x.value = ’Diesel’

del x.value

Exit:

 Getting value Peter Setting value to Diesel Deleting value 

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Descriptor in Python __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

Descriptor in Python __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]

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