Top 15 important Python Topics

Top 15 important Python Topics

1.Class Attributes vs Instance Attributes in Python

Class Attributes: Class attributes are shared among all instances of a class. They are defined within the class but outside any methods using the class keyword. They have the same value for every instance of the class.

python
class MyClass:
    class_attribute = 10

obj1 = MyClass()
obj2 = MyClass()

print(obj1.class_attribute)  # Output: 10
print(obj2.class_attribute)  # Output: 10

Instance Attributes: Instance attributes are specific to each instance of a class. They are defined within the class's methods using the self keyword and have different values for each instance.

python
class MyClass:
    def __init__(self, value):
        self.instance_attribute = value

obj1 = MyClass(5)
obj2 = MyClass(7)

print(obj1.instance_attribute)  # Output: 5
print(obj2.instance_attribute)  # Output: 7

2. Remove duplicate items from a list in Python

To remove duplicate items from a list, you can convert the list into a set to automatically eliminate duplicates and then convert it back to a list.

python
my_list = [1, 2, 2, 3, 4, 4, 5]
unique_list = list(set(my_list))

print(unique_list)  # Output: [1, 2, 3, 4, 5]

3. Remove items from a list in Python

To remove specific items from a list, you can use the remove() method.

python
my_list = [1, 2, 3, 4, 5]
my_list.remove(3)

print(my_list)  # Output: [1, 2, 4, 5]

4. repr() vs str() in Python

repr() returns a printable representation of an object, which can be used to recreate the object. str(), on the other hand, returns a human-readable string representation of an object.

python
class MyClass:
    def __repr__(self):
        return "MyClass()"

    def __str__(self):
        return "Instance of MyClass"

obj = MyClass()

print(repr(obj))  # Output: MyClass()
print(str(obj))   # Output: Instance of MyClass

5. How to sort a dictionary by value in Python?

You can use the sorted() function along with a lambda function as the key parameter to sort the dictionary by its values.

python
my_dict = {'apple': 5, 'banana': 2, 'orange': 8}
sorted_dict = dict(sorted(my_dict.items(), key=lambda item: item[1]))

print(sorted_dict)  # Output: {'banana': 2, 'apple': 5, 'orange': 8}

6. globals() and locals() Method in Python

globals() returns a dictionary representing the global symbol table, and locals() returns a dictionary representing the current local symbol table.

python
global_var = 10

def my_function():
    local_var = 20
    print(globals())  # Output: {'__name__': '__main__', 'global_var': 10, ...}
    print(locals())   # Output: {'local_var': 20}
    
my_function()

7. Convert User Input to a Number

You can use the input() function to get user input, and then use type casting to convert it to a specific numeric type like int or float.

python
user_input = input("Enter a number: ")
number = int(user_input)

print(number)  # Output: (User input as an integer)

8. Convert String to Datetime in Python

You can use the strptime() method from the datetime module to convert a string to a datetime object.

python
from datetime import datetime

date_string = "2023-07-30"
date_object = datetime.strptime(date_string, "%Y-%m-%d")

Topic 9: How to Call External Commands in Python?

Introduction:
In Python, you can call external commands using the subprocess module. This module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes.

Code Example:

python
import subprocess

# Example command: ls (list files in the current directory)
command = "ls"

# Run the command and capture the output
result = subprocess.run(command, shell=True, capture_output=True, text=True)

# Print the output
print(result.stdout)

Explanation:

• We import the subprocess module to use its functionalities for calling external commands.
• In this example, we call the "ls" command using subprocess.run().
• The shell=True parameter allows us to use shell features, and capture_output=True captures the command's output.
• The text=True parameter converts the output to a string for easier handling.
• The result is then printed using print(result.stdout).

Topic 10: How to Count the Occurrences of a List Item?

Introduction:
To count the occurrences of a specific item in a list, you can use the count() method.

Code Example:

python
# Example list
my_list = [1, 2, 2, 3, 4, 2, 5]

# Count occurrences of '2'
count_of_2 = my_list.count(2)

# Print the count
print("Count of 2:", count_of_2)

Explanation:

• We have a list called my_list containing various elements, including multiple occurrences of '2'.
• Using the count() method, we find the number of occurrences of '2' and store it in the variable count_of_2.
• The count is then printed using print().

Topic 11: How to Flatten List in Python?

Introduction:
Flattening a nested list means converting it into a single-dimensional list. You can achieve this using various approaches, like using list comprehension or the itertools.chain() method.

Code Example:

python
# Example nested list
nested_list = [[1, 2], [3, 4], [5, 6]]

# Method 1: Using list comprehension
flattened_list = [item for sublist in nested_list for item in sublist]

# Method 2: Using itertools.chain()
import itertools
flattened_list2 = list(itertools.chain(*nested_list))

# Print the flattened lists
print("Flattened list (Method 1):", flattened_list)
print("Flattened list (Method 2):", flattened_list2)

Explanation:

• We start with a nested list called nested_list.
• Method 1: We use list comprehension to iterate through each sublist and each item within the sublists, creating a single list flattened_list.
• Method 2: We import the itertools module and use the chain() method to combine the sublists into a single iterable. We convert the iterable into a list to get flattened_list2.
• Both flattened lists are then printed using print().

Topic 12: How to Merge Dictionaries in Python?

Introduction:
In Python, you can merge dictionaries using the update() method or dictionary unpacking (Python 3.5+).

Code Example:

python
# Example dictionaries
dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}

# Method 1: Using update() method
merged_dict1 = dict1.copy()
merged_dict1.update(dict2)

# Method 2: Using dictionary unpacking (Python 3.5+)
merged_dict2 = {**dict1, **dict2}

# Print the merged dictionaries
print("Merged dictionary (Method 1):", merged_dict1)
print("Merged dictionary (Method 2):", merged_dict2)

Explanation:

• We have two dictionaries, dict1 and dict2, containing key-value pairs.
• Method 1: We create a copy of dict1 and use the update() method to merge dict2 into merged_dict1.
• Method 2: We use dictionary unpacking ({**dict1, **dict2}) to merge the dictionaries and create merged_dict2.
• Both merged dictionaries are then printed using print().

Topic 13: How to Pass Value by Reference in Python?

Introduction:
In Python, objects are passed to functions by reference by default. This means that when you pass an object (e.g., list or dictionary) to a function and modify it inside the function, the changes are reflected outside the function as well.

Code Example:

python
def modify_list(my_list):
    my_list.append(4)

# Example list
original_list = [1, 2, 3]

# Call the function with the list
modify_list(original_list)

# Print the modified list
print("Modified list:", original_list)

Explanation:

• We define a function modify_list() that takes a list as an argument and appends '4' to it.
• We have an example list called original_list.
• When we call the modify_list() function with original_list, the list is modified inside the function.
• After the function call, we print original_list and see that it has been modified.

Topic 14: Compare Strings in Python

Introduction:
You can compare strings in Python using comparison operators like ==, !=, <, >, <=, and >=. These operators perform lexicographic (dictionary) comparisons based on the ASCII values of characters.

Code Example:

python
# Example strings
string1 = "apple"
string2 = "banana"

# Compare strings using comparison operators
is_equal = string1 == string2
is_not_equal = string1 != string2
is_less_than = string1 < string2
is_greater_than = string1 > string2

# Print the comparison results
print("Equal:", is_equal)
print("Not Equal:", is_not_equal)
print("Less Than:", is_less_than)
print("Greater Than:", is_greater_than)

Explanation:

• We have two example strings, string1 and string2.
• We use various comparison operators to compare the strings and store the results in different variables.
• The comparison results are then printed using print().

Topic 15: Convert File Data to List

Introduction:
To convert data from a file into a list in Python, you can open the file, read its content, and split it into individual elements to create a list.

Code Example:
Suppose we have a file named "data.txt" with the following content:

apple
banana
orange

python
# Read data from the file and convert to list
with open("data.txt", "r") as file:
    data_list = file.read().splitlines()

# Print the list
print("Data List:", data_list)

Explanation:

• We open the file "data.txt" in read mode using the open() function and use a with block to ensure the file is properly closed after reading.
• We read the file content using file.read() and split it into lines using .splitlines(), creating data_list.
• The list is then printed using print(). The output will show the list containing the lines from the file as its elements.