02-09: Exercises — Boolean and None

Notes reference: 02-09: Boolean and None Types

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Q1: bool arithmetic

True and False are subclasses of int. Predict and verify the output of:

True + True
False + 10
True * 7
True + False + True

Solution

print(True + True)          # 2
print(False + 10)           # 10
print(True * 7)             # 7
print(True + False + True)  # 2

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Q2: Logical operators truth table

Fill in the results:

A	B	A and B	A or B	not A
True	True
True	False
False	True
False	False

Solution

for A in [True, False]:
    for B in [True, False]:
        print(f"{str(A):<5} {str(B):<5} | {str(A and B):<5} | {str(A or B):<5} | {str(not A):<5}")

True  True  | True  | True  | False
True  False | False | True  | False
False True  | False | True  | True
False False | False | False | True

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Q3: Truthy and falsy

Classify each value as truthy or falsy: 0, "", " ", [], [0], None, 0.0, "False", False

Solution

values = [0, "", " ", [], [0], None, 0.0, "False", False]
for v in values:
    print(repr(v), "→", bool(v))

0       → False
""      → False
" "     → True    (non-empty string!)
[]      → False
[0]     → True    (non-empty list!)
None    → False
0.0     → False
"False" → True    (non-empty string!)
False   → False

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Q4: Short-circuit evaluation

Predict the output without running the code. Explain why no error occurs on line 2.

x = 0
safe = (x != 0) and (1 / x > 0.5)
print(safe)

Solution

safe = False
No ZeroDivisionError because `and` short-circuits:
once (x != 0) evaluates to False, (1 / x > 0.5) is never evaluated.

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Q5: or as default value

Write a one-liner that assigns "Guest" to display_name if username is an empty string, otherwise uses username.

Solution

username     = ""
display_name = username or "Guest"
print(display_name)   # Guest

username     = "Jahid"
display_name = username or "Guest"
print(display_name)   # Jahid

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Q6: Comparison chaining

Check whether score = 85 is in the grade B range (80 ≤ score < 90) using a chained comparison.

Solution

score = 85
is_B = 80 <= score < 90
print(is_B)   # True

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Q7: None checks

Write code that sets result = None, then checks if it is None using the correct operator.

Solution

result = None

if result is None:
    print("No result yet")

# Incorrect (but works) — avoid this:
# if result == None: ...

# Also correct — checking if it has a value:
if result is not None:
    print("Result:", result)
else:
    print("Result is absent")

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Q8: bool() conversions

Use bool() to show that non-zero numbers are truthy and that all "empty" containers are falsy.

Solution

print(bool(42))      # True
print(bool(-1))      # True
print(bool(0))       # False

print(bool("hi"))    # True
print(bool(""))      # False

print(bool([1, 2]))  # True
print(bool([]))      # False

print(bool(None))    # False

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⬅️ Previous: 02-08: Exercises — String Formatting ➡️ Next: 02-10: Exercises — Type Conversion