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Basic Operations with Variables

Basic Operations with Variables in Python

Python provides various operations and methods for working with data and performing various tasks. Let's review the most popular which relate to all variable types.

Type Definition

There are several ways to check the type of a variable in Python. Let's see how to check a type of a variable in Python.

Function type()

This built-in type() function returns the data type of a variable, it is the equivalent of typeof in other programming languages.

Let's see how to print type of variable in Python:

x = 23
print(type(x))   # Output: <class 'int'>

y = 3.14
print(type(y))   # Output: <class 'float'>

z = "Hello, friend!"
print(type(z))   # Output: <class 'str'>

w = True
print(type(w))   # Output: <class 'bool'>

In this example, the type() function is used to determine the type of variables x, y, z, and w. The output shows the class of the variable.

Function isinstance()

This built-in function checks whether a variable is an instance of a specified class or not. With this function we can easily check if a variable is string or not.

So here we are checking type of variable in Python:

x = 5
print(isinstance(x, int))  # Output: True

y = "hello"
print(isinstance(y, str))  # Output: True

z = [1, 2, 3]
print(isinstance(z, list))  # Output: True

Using the __class__ Attribute

This attribute returns the class of an object.

Example:

x = 17
print(x.__class__)  # Output: <class 'int'>

y = "hello"
print(y.__class__)  # Output: <class 'str'>

z = [1, 2, 3]
print(z.__class__)  # Output: <class 'list'>

This example shows how to print a type of a variable in Python.

Using the type() Function with an assert Statement

This is a way to check the type of a variable and raise an error if the type is not as expected.

Example:

x = 98
assert type(x) == int

y = "hello"
assert type(y) == str

z = [1, 2, 3]
assert type(z) == list

These are ways how to get a type of a variable in Python, for example, you can check if a variable is a list (see the example above).

Deleting Variables

Using the del Statement

In Python, the del statement is used to delete a variable or an item from a list, dictionary or any other collection object. The syntax for using del is:

del object

Where object can be a variable, a list item, a dictionary item, or any other object.

Here are a few examples:

# Delete a variable
x = 5
del x

# Delete an item from a list
my_list = [1, 2, 3, 4, 5]
del my_list[2]

# Delete an item from a dictionary
my_dict = {'a': 1, 'b': 2, 'c': 3}
del my_dict['b']

In the first example, the variable x is deleted using the del statement. After the statement is executed, the variable x no longer exists.

In the second example, the third item in the list my_list is deleted using del my_list[2]. After the statement is executed, my_list contains the items [1, 2, 4, 5].

In the third example, the item with key 'b' is deleted from the dictionary my_dict using del my_dict['b']. After the statement is executed, my_dict contains the items {'a': 1, 'c': 3}.

Note that del only deletes the reference to the object, not the object itself. If the object is still referenced elsewhere in the code, it will not be deleted from memory.

Assigning None to the Variable

One more method to clear a variable is to assign the value None to a variable to indicate that it has no value. The None value is a built-in constant that represents the absence of a value. Here's an example:

x = None
print(x)  # Output: None

In this example, we assign the value None to the variable x. When we print the value of x, we see that it outputs None.

Using the locals() or globals() Functions

You can use the locals() and globals() functions to access the local and global namespace, respectively. These functions return a dictionary that contains all the variables and their values in the current namespace. You can use these functions to remove variables from the namespace by using the del statement.

Here's an example:

def my_function():
    x = 5
    y = 10
    del locals()['y']
    print(x)
    print(y)  # This will raise a NameError

my_function()

In this example, we define a function my_function() that creates two variables x and y. We then use the del statement to remove the variable y from the local namespace by using del locals()['y']. After the statement is executed, y is no longer defined in the local namespace, so when we try to print its value, we get a NameError.

You can also use the globals() function to remove variables from the global namespace. Here's an example:

x = 5
y = 10

def my_function():
    del globals()['x']

my_function()

print(x)  # This will raise a NameError
print(y)  # Output: 10

In this example, we define two variables x and y in the global namespace. We then define a function my_function() that uses del globals()['x'] to remove the variable x from the global namespace. After the function is called, x is no longer defined in the global namespace, so when we try to print its value, we get a NameError. However, y is still defined in the global namespace, so we can print its value without any errors.

In addition, you can clear all variables from the current namespace by using the globals() and locals() functions. Here's how you can do it:

# Clear all global variables
for var in globals().copy():
    if var.startswith("__"):
        continue
    del globals()[var]

# Clear all local variables
for var in locals().copy():
    if var.startswith("__"):
        continue
    del locals()[var]

The above code will delete all variables from the global and local namespaces except for the built-in Python variables (which start with __). Keep in mind that this can be dangerous as it will delete all your variables and you won't be able to access them later in your program. So, make sure you use it with caution.

Using the __del__() Method

The __del__() method is a special method that is called when an object is about to be destroyed or garbage collected. You can define this method in your class to perform any cleanup tasks that are required before the object is destroyed.

The __del__() method takes no arguments and has no return value. Here's an example:

class MyClass:
    def __init__(self, name):
        self.name = name
    
    def __del__(self):
        print(f"{self.name} has been deleted")
        
obj1 = MyClass("Object 1")
obj2 = MyClass("Object 2")
del obj1

In this example, we define a class MyClass with an __init__() method that initializes an instance variable name. We also define an __del__() method that prints a message when the object is deleted. We create two instances of the class MyClass and then delete one of them using the del statement.

When we run this code, we get the following output:

Object 1 has been deleted

This output indicates that the __del__() method was called when the object obj1 was deleted.

Note that the __del__() method is not guaranteed to be called in all cases. For example, if the program terminates abruptly, the method may not be called. Therefore, it's generally not a good idea to rely on this method for critical cleanup tasks. Instead, it's better to use other cleanup mechanisms, such as with statements or context managers, where possible.

Operations with Variables

We have already covered the assignment of variables earlier. But let's also see what else we can do with variables.

Copying a variable

To copy variable in Python you can use assignment operator = or the copy() method. However, it is important to note that the behavior of these methods can be different depending on the type of the variable.

  • Shallow copy: When you use the assignment operator to copy a list, dictionary, or other mutable objects, you create a shallow copy. This means that the new variable refers to the same object in memory as the original variable. Any changes made to the original variable will be reflected in the copy, and vice versa. For example:
original_list = [1, 2, 3]
copy_list = original_list  # Shallow copy
original_list[0] = 4
print(copy_list)  # Output: [4, 2, 3]
  • Deep copy: If you want to create a new object in memory that is a copy of the original variable, you can use the copy() method. This creates a deep copy, which means that any changes made to the original variable will not be reflected in the copy, and vice versa. For example:
import copy

original_list = [1, 2, 3]
copy_list = copy.deepcopy(original_list)  # Deep copy
original_list[0] = 4
print(copy_list)  # Output: [1, 2, 3]

It is important to use the appropriate method for copying variables in order to avoid unexpected behavior in your code.

Check if a Variable is Defined or not

You can use the in operator to check if a variable has been defined or not.

Here's an example:

if 'my_variable' in locals() or 'my_variable' in globals():
    print('my_variable is defined')
else:
    print('my_variable is not defined')

This code checks if the variable my_variable is defined in either the local or global namespace. If it is defined, the code will print 'my_variable is defined', otherwise it will print 'my_variable is not defined'.

Note that if the variable has been defined but has a value of None, this method will still consider it as defined. If you want to check specifically for a non-None value, you can modify the code like this:

if 'my_variable' in locals() and locals()['my_variable'] is not None or 'my_variable' in globals() and globals()['my_variable'] is not None:
    print('my_variable is defined and has a non-None value')
else:
    print('my_variable is not defined or has a value of None')

This code checks both that the variable is defined and that it has a non-None value. If both conditions are met, it will print 'my_variable is defined and has a non-None value', otherwise it will print 'my_variable is not defined or has a value of None'.

Check if a Variable is Empty

In Python, there are several ways to check if a variable is empty:

  • Using the if statement with the variable name:
if my_variable:
    # do something
    pass
else:
    # the variable is empty
    pass

If my_variable is empty or evaluates to False, the else block will be executed.

  • Using the not operator with the variable name:
if not my_variable:
    # the variable is empty
    pass
else:
    # do something
    pass

If my_variable is empty or evaluates to False, the if block will be executed.

  • Using the built-in len() function:
if len(my_variable) == 0:
    # the variable is empty
    pass
else:
    # do something
    pass

If my_variable has a length of 0, it is considered empty.

  • Using the is operator with None:
if my_variable is None:
    # the variable is empty
    pass
else:
    # do something
    pass

If my_variable is equal to None, it is considered empty. However, it's important to note that None is a specific value in Python and is not the same as an empty string, list, or other data types.

Getting the Name of a Variable

Variables are simply names that refer to objects in memory. In Python you can get the name of a variable using the locals() or globals() functions, depending on whether the variable is in the local or global namespace.

Here's an example:

x = 42
var_name = [k for k,v in locals().items() if v is x][0]
print(var_name)

This code assigns the value 42 to the variable x. Then it uses a list comprehension to iterate over all the items in the local namespace, and find the variable name that refers to the object with the value of x. Finally, it prints out that variable name, which in this case is x.

Note that this approach assumes that the variable has a unique value in the namespace, which may not always be the case. Also, it's generally not recommended to rely on variable names in your code logic, as it can make your code more brittle and harder to refactor.

Check if a Variable Exists

You can check if a variable exists in Python by using the in keyword along with the locals() or globals() function. Here is an example:

if 'my_variable' in locals():
    print("my_variable exists in the local scope.")

if 'my_variable' in globals():
    print("my_variable exists in the global scope.")

In the above example, we check if a variable named my_variable exists in either the local or global scope using the in keyword with the locals() or globals() function, respectively. If the variable exists, we print a message indicating that it exists in the specified scope.

Note that checking for the existence of a variable using the in keyword and locals() or globals() function will only work for variables that have already been defined in the program. If you are unsure if a variable has been defined or not, you can use a try-except block to catch a NameError exception that would be raised if the variable does not exist. Here is an example:

try:
    my_variable
    print("my_variable exists.")
except NameError:
    print("my_variable does not exist.")

In this example, we try to access the my_variable variable, and if it does not exist, a NameError exception is raised. We catch the exception using the except block and print a message indicating that the variable does not exist.

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Updated: 02/11/2024 - 18:15
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