Unit 2 · Foundational
Decomposition, and the scope rules that make it safe. This is the unit that decides whether later programs stay readable as they grow.
What a strong answer looks like
A strong Unit 2 answer states what a function takes, what it returns, and what it leaves unchanged, before any of its body.
Know
Python passes references to objects. Rebinding a parameter inside a function does not affect the caller, but mutating a mutable argument does.
Apply
Decide for each parameter whether the function may modify it, and document that.
Watch out
Mutating a list argument unintentionally, which changes the caller data invisibly.
Study move
Write one function that fails to change a number and one that changes a list, and explain both.
Know
A function without an explicit return yields the null object. Returning is different from printing, and only one of them is usable by the caller.
Apply
Return results and let the caller decide whether to display them.
Watch out
Printing inside a function then trying to use the result, which silently gives nothing.
Study move
Convert a printing function into a returning one and update its caller.
Know
Names assigned inside a function are local unless declared otherwise. Reading an outer name works; assigning to it creates a new local one.
Apply
Pass values in and return them out rather than reaching for globals.
Watch out
Assigning to a name that also exists outside, which creates a local and leaves the outer one untouched.
Study move
Write a function that appears to modify a global counter but does not, then fix it properly by returning.
Know
Defaults are evaluated once when the function is defined, so a mutable default is shared across every call.
Apply
Use an immutable sentinel as the default and create the mutable value inside the body.
Watch out
A mutable default argument accumulating values between calls, which is one of the most confusing bugs in the language.
Study move
Write a function with a mutable default, call it three times, and explain the growing result.
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 function that takes a list of scores and returns the top three without modifying the original list, then prove the original is unchanged.
