Unit 1 · Foundational
Variables, types, operators, and control flow. Python hides type declarations, which makes the language friendly to start and makes type errors appear later than they would elsewhere.
What a strong answer looks like
A strong Unit 1 answer names the type of each value before reasoning about the result, because dynamic typing means the type is not written down anywhere.
Know
A Python variable is a name bound to an object, and the type belongs to the object rather than to the name. Rebinding a name to a different type is legal.
Apply
Track what each name currently refers to, especially after reassignment inside a branch.
Watch out
Assuming a name keeps its original type. Nothing prevents it changing, and the error surfaces far from the cause.
Study move
Rebind one name to three different types and print the type after each.
Know
The two division operators differ: one always produces a float, the other floors toward negative infinity. Mixing int and float promotes to float.
Apply
Choose floor division when counting whole groups and true division when a fraction is meant.
Watch out
Floor division of a negative number rounds toward negative infinity, not toward zero.
Study move
Evaluate both division operators on a negative operand and explain the difference.
Know
Strings are immutable sequences. Formatted string literals interpolate expressions directly and are the clearest way to build output.
Apply
Build messages with formatted literals rather than repeated concatenation.
Watch out
Expecting a string method to modify in place. Every one returns a new string.
Study move
Rewrite a concatenated message as a formatted literal and compare readability.
Know
Indentation defines blocks, so structure is enforced by layout. A for loop iterates over a sequence directly rather than over indices.
Apply
Iterate over items when the index is not needed, and enumerate when it is.
Watch out
Looping over a range of indices out of habit, which is longer and easier to get wrong than iterating directly.
Study move
Rewrite an index-based loop as a direct iteration and then as an enumeration.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Write a program that converts a total number of minutes into hours and minutes and prints a formatted sentence, then test it on 0, 59, and 60.
