Python Syntax

lists

• Python lists are mutable
• This is problematic when you are passing lists as a argument in a function

my_list = []
new_list = my_list # creates a pointer to my_list
 
my_list.append(2)
print(new_list) # [2]
 
copied_list = my_list.copy() # creates a new list object in memory
 
my_list.append(3)
print(my_list) # [2, 3]
print(new_list) # [2, 3]
print(copied_list) # [2] 
 
# sort in reverse order
my_list.sort(reverse=True)

Set

seen = set() 
seen.add(1)
seen.remove(1)

• can compare equality of two sets with ==

Dictionary

my_dict = {}
my_dict["key"] = "value"
 
# to force a default value 
my_dict.get("key", "default value")
 
#remove 
my_dict.pop("key")

String

my_string = " my string " 
 
# remove trailing whitespace
my_string.strip()
 
# convert into array based on space
my_string.split(" ")

Python List as stacks

my_stack = []
my_stack.append(10) # [10]
my_stack.append(20) #[10, 20]
my_stack[-1] # look at the last element of the stack 
 
print(my_stack.pop()) # 20 (last element added into stack)

Double-ended queue in python

my_queue = collections.deque([])
 
my_queue.append(10) # Appends 10 to the right of the queue
my_queue.appendleft(20) # Appends 20 to the left of the queue
# [20, 10]
 
my_queue.pop() # pops from the right of the queue
# [20]
 
my_queue.append(10)
# [10, 20]
 
my_queue.popleft() # pops from the left of the queue
# [10]
 
my_queue.append(10)
# [10, 20]
 
# Accessing the queue
print(my_queue[0]) # 10
print(my_queue[-1]) # 20
 

List Comprehension

[expression for item in iterable if condition]
 
numbers = range(10) # 0 ... 9
even_numbers = [num for num in numbers if numbers % 2 == 0]
 
 

DefaultDicts

from collections import defaultdict 
 
# Using list as default_factory  
word_counts = defaultdict(list)  
word_counts['fruits'].append('apple')  
word_counts['fruits'].append('banana')  
word_counts['vegetables'].append('carrot')  
print(word_counts)  
# Output: defaultdict(<class 'list'>, {'fruits': ['apple', 'banana'], 'vegetables': ['carrot']})
 
# Using int as default_factory  
char_counts = defaultdict(int)  
for char in "hello":  
char_counts[char] += 1  
print(char_counts)  
# Output: defaultdict(<class 'int'>, {'h': 1, 'e': 1, 'l': 2, 'o': 1})

• list: New keys will have an empty list [] as their default value. This is useful for grouping items.
• int: New keys will have 0 as their default value. Useful for counting occurrences.
• set: New keys will have an empty set {} as their default value.
• str: New keys will have an empty string "" as their default value.

f string

name = "Alice"
age = 30
message = f"Hello, {name}. You are {age} years old."
print(message)
# Output: Hello, Alice. You are 30 years old.
 
result = f"The sum of 5 and 10 is {5 + 10}."
print(result)
# Output: The sum of 5 and 10 is 15.
 
price = 123.45678
formatted_price = f"The price is {price:.2f}."  # Format to 2 decimal places
print(formatted_price)
# Output: The price is 123.46.
 
large_number = 1234567
formatted_number = f"Value: {large_number:,}" # Add comma as thousands separator
print(formatted_number)
# Output: Value: 1,234,567

Assert

# assert condition, message
 
def add(a, b):
	return a + b 
 
assert add(1, 1) == 2, "expected 2 but got something else"

• if condition evaluates to False, an AssertionError is raised
• message is optional but provides additional information about the assertion if it fails

tricks

 
heights = [10, 10, 10, 10]
 
for i, h in enumerate (heights):
	# i represents the index
	# h represents the height at each index

• nonlocal keyword is used within nested functions to explicitly declare that a variable refers to a variable in the nearest enclosing scope that is not the global scope