Pass Values and Return a Result

Turn the fixed summary into mean(readings), bind an argument to its parameter, and return either a numerical mean or None when no reading is available.

show_summary() always uses the readings written inside its body. To summarize a different measurement, the function needs an input supplied by its caller.

Pass Readings into the Function

Put the name readings between the parentheses in the definition:

Here readings is a parameter: a local name that receives an input for one call. A call supplies an argument:

For this call, the value referred to by morning_readings is passed to the parameter readings. The function performs the familiar loop with that value.

Return the Calculated Result

The return statement sends a value back to the code that made the call. Trace the morning call from input to result:

Step	Current reading	`total`	`count`	Result sent to caller
bind argument to `readings`	—	—	—	—
initialize summary	—	0	0	—
process 18	18	18	1	—
process 21	21	39	2	—
process 20	20	59	3	—
process 23	23	82	4	—
return `total / count`	—	82	4	20.5

After the return, morning_mean refers to 20.5. The caller can store, compare, or display that value:

print(f"Morning mean: {morning_mean:.1f} °C")

Q1. Trace an argument and return

What value is assigned to result?

result = mean([18, None, 22])

Use the mean definition above.

Returned mean

Compute it first, then check your number.

HintCount only available readings

The total is 40 and the count is 2.

SolutionThe call returns twenty

The loop processes 18 and 22, giving total = 40 and count = 2. The return expression sends 20 back to result.

Printing Is Not Returning

This function displays a value but does not return that value:

If a function reaches the end of its body without an explicit return, Python returns None:

The output is:

Mean: 20.5 °C
None

The first line is displayed inside show_mean. The second line displays the value stored in shown. Printing is an action for a reader; returning provides a value that the surrounding program can use.

Q2. Separate display from return

What value is assigned to stored?

Choose one

Count: 4

None

Select one choice, then check.

HintLook for a return statement

The function body contains only print().

SolutionThe call returns None

show_count(4) displays a line and then reaches the end of its body. Python returns None, so stored refers to None.

Define the No-Reading Result

The first mean definition assumes that at least one reading is available. An empty list or a list containing only None leaves count at zero, so division cannot produce a mean.

Handle that case before the ordinary return:

The first return is an early return. When no available reading exists, it sends None to the caller and ends that call. Otherwise Python continues to the ordinary mean calculation.

Now the same function has a clear result for several datasets:

None means that no numerical mean is available. It does not mean that the mean is zero.

Q3. Repair the unavailable result

Repair mean so the program displays 20.0 and then None.

Editable Python

HintPreserve the meaning of absence

Change the first return value. Zero is a numerical result; None records that no numerical mean exists.

SolutionReturn None for no available readings

A parameter receives the caller's input, and return sends a result back. mean now handles ordinary and unavailable data; next we will trace the fresh local work created by each call.