Week 1: Basics and Expressions

Course: Jan 2026 - Python Focus: Types, arithmetic, strings, and the first trap layer of Python expressions.

1. Core idea

What to remember

• int, float, str, and bool are the base types.
• input() always returns a string.
• / gives a float, even if the result looks whole.
• // is floor division, not normal division.
• Strings are sequences, so indexing and slicing work on them.

2. Common traps

• 5 / 2 is 2.5, not 2.
• "5" + 2 is invalid, because string and integer are different types.
• and / or return one of the operands, not always True or False.
• s[2] works only if s is a sequence.

3. Speedrun pattern

1. What type is each operand?
2. Does the operator return a value or a type?
3. Is this expression valid before evaluating its result?

4. Micro revision

• Type check: type(x)
• Cast: int(...), float(...), str(...)
• Sequence access: s[i], s[a:b:c]

Assignment anchor

• Week 1 Graded Assignment 1

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