Functions Lab

Call visualiser: frames appear, values go in, values come back

Every example from the lecture. Watch Python skip the function body at def, open a new frame at each call, and remove it at return.

What to notice

def only stores the function. Its body runs at the call.
Each call gets a fresh frame. Its names disappear when the call ends.
A function without return gives back None.
Untick "numbers and text inside the frame" to see that every name points to an object.

Try this

In print or return?, predict the values of a and b before running. In Functions calling functions, how many frames are open at the busiest moment? In Local names stay inside, change the last line so it prints the greeting without an error. Open The default-list trap. Why does the second call print two 1s?

Argument matcher: which value goes into which parameter?

Pick a function, type a call, and see how Python matches arguments to parameters: by position, by keyword, or from a default. Wrong calls show the real error.

Function definition Call

How the arguments are matched

What to notice

Positional arguments fill parameters left to right. Keywords fill by name, in any order.
Positional arguments must come before keyword arguments in a call.
A default is used only when nothing else filled that parameter.

Try this

Call printinfo so that age is 50 without writing age=. Produce each of the three errors from the "rules for arguments" slide. Choose def f(age=35, name):. Why does Python refuse before any call?

Change the object, or move the name?

A parameter is a second name for the caller's object. Choose what the function does with it, predict whether the caller sees a change, then check.

The caller passes

Inside the function

After the call, does outside look different?

Your results so far

What to notice

Methods like append and item assignment change the object, so every name for it sees the change.
lst = ... or n = n + 1 only moves the local name. The caller's name still points to the old object.
Numbers and text can never be changed in place. To send a new value out, return it.
Surprise: lst += [100] changes the list in place, but lst = lst + [100] does not.

Try this

Predict correctly for all six list operations. Explain in one sentence why s.upper() does not change the caller's text. Which choices make the caller's value change only because of return?

Write and test: functions that return their answer

The in-class exercises and homework, with automatic tests. The expected answers were produced by running a correct solution in real Python.

Exercise

Test results

Printed while testing

What to notice

The tests check what your function returns, not what it prints.
"got None" usually means a missing return, or a return inside the loop that ends it too early.
Your code is saved in this browser, so you can come back to it.

Try this

Pass every test in evens, gcd and lcm. Make mex pass, then explain why mex([]) is 0. When a test fails, press Step through test 1 and find the line where it goes wrong.

Works offline: no internet needed. The Python shown here runs in your browser with the same rules as Python 3, so results and error messages match what you will see in Jupyter.

Call visualiser: frames appear, values go in, values come back

What to notice

Try this

Argument matcher: which value goes into which parameter?

How the arguments are matched

What to notice

Try this

Change the object, or move the name?

1. At the call

2. After the line inside the function

3. Back outside, after the call

Your results so far

What to notice

Try this

Write and test: functions that return their answer

Test results

Printed while testing

What to notice

Try this