Cheat sheet for Python interview

1. Swap variables without a temporary one

a, b = 5, 10
a, b = b, a

2. One-line if-else (ternary)

result = "Even" if x % 2 == 0 else "Odd"

3. List Comprehension

squares = [x**2 for x in range(10)]
evens = [x for x in range(10) if x % 2 == 0]

4. Set and Dict Comprehension

unique = {x for x in [1,2,2,3]}        # remove duplicates
squares = {x: x**2 for x in range(5)}  # dict comprehension

5. Most common element in a list

from collections import Counter
most_common = Counter(['a','b','a','c']).most_common(1)[0][0]

6. Merging dictionaries (Python 3.9+)

a = {'x': 1}
b = {'y': 2}
merged = a | b

7. Returning multiple values

def stats(x):
    return max(x), min(x), sum(x)

high, low, total = stats([1, 2, 3])

8. Using zip to iterate over two lists

names = ['a', 'b']
scores = [90, 85]

for n, s in zip(names, scores):
    print(f"{n}: {s}")

9. Flattening nested lists

nested = [[1,2], [3,4]]
flat = [item for sublist in nested for item in sublist]

10. Default values in a dictionary

from collections import defaultdict
d = defaultdict(int)
d['apple'] += 1   # no KeyError

11. Lambda in one line

square = lambda x: x**2
print(square(4))

12. enumerate with index

for i, v in enumerate(['a', 'b', 'c']):
    print(i, v)

13. Sorting by key or value

d = {'a': 3, 'b': 1, 'c': 2}
sorted_by_val = sorted(d.items(), key=lambda x: x[1])

14. Reading file lines into a list

with open('file.txt') as f:
    lines = f.read().splitlines()

15. Type Hints

def add(x: int, y: int) -> int:
    return x + y

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