python-week1

Week map

Type notation

• int → integer type → whole numbers → 17, -4, 0.
• float → floating type → decimal approximations → 3.0, -0.25, 2.718.
• bool → boolean type → truth values → True, False only.
• str → string type → text sequence → "score", 'A'.
• type(x) → returns type of value → type(3/2) is float in Python 3.

Literals and assignment

• x = 5 → assignment statement → bind name x to value 5 → later x reads 5 until reassigned.
• = is not equality test; equality is ==.

python
Run Inline
a = 10
b = a + 3
a = 2
print(b)

Expressions and operators

Algorithm tracing habit

python
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x = 4
y = x * 2
x = y - 1
print(x + y)

Pattern families

Easy — Final value prediction

• Straight-line assignments without branches.
• Single print of expression or variable.
• Identify type of literal or simple expression.

Medium — Mixed-type expressions

• Division always float in Python 3.
• Floor division and mod with negatives (know course convention; often positive mod for quiz).
• String + vs numeric + — types must match for numeric add.

Hard — Multi-variable dependency chains

• Reassignment overwrites; old value gone unless another name still references it (week 1 usually one name per value).
• Expression on RHS fully evaluated before LHS update.
• print with comma-separated items vs concatenation.

Worked mini-examples

python
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a = 9 / 2      # 4.5 float
b = 9 // 2     # 4 int
c = 9 % 2      # 1

python
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n = 7
msg = "Level " + str(n)   # "Level 7"
# "Level " + n  would error

python
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result = 10 - 2 ** 3 + 1   # 10 - 8 + 1 = 3

python
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p = 1
q = p + 4    # q = 5
p = q        # p = 5
q = p + 2    # q = 7
# p is 5, q is 7

Traps

• Using = in a condition (week 2 topic, but appears early in distractors).
• Assuming / gives integer quotient in Python 3.
• Concatenating str with int without str().
• Reading code bottom-up when state evolves top-down.
• Confusing // with /.

Diagnostic (try yourself)

1. What is printed?

python
Run Inline
x = 8
y = x // 3
x = y * 5
print(x)

1. What is the type of 3 + 2.0? What is the type of 4 / 2?
2. What is the value of 10 - 3 ** 2 + 1?
3. After a = 6, b = a, a = 10, what are a and b?
4. Write one expression that produces "Go" + "Go" using only the integer 2 and string operations (no quotes in the expression except on "Go").