Read and Predict a Small Program

The temperature program contains six instructions. It has no choices or repeated steps, so Python runs these instructions from top to bottom, one at a time. Following that order lets us predict the result before we run the code.

Predict the Report

Here is the program again:

The first line calculates 82 and remembers that result under the name total. The second remembers 4 under count. The third uses those remembered results to calculate 20.5 and remembers it under mean.

The next three instructions display the report. We can therefore predict:

total: 82
count: 4
mean: 20.5

Run the program only after making this prediction.

When predicted and actual output agree, we have evidence that we followed this run correctly. It does not prove that every possible program would work, but it does check our reading of this one.

Follow One Instruction at a Time

The following trace keeps each instruction beside the change it causes:

The first three instructions calculate values without displaying them. Output appears only when the program reaches print(). A function is named code that performs a task; print() is a built-in Python function whose task is to display values.

The line-by-line trace is also a useful reading method. Keep three questions separate:

1. Which instruction runs now?
2. What result does it calculate or remember?
3. Does it add anything to the output?

Source and Output Must Stay Separate

Consider this instruction:

print("mean:", mean)

It contains a function name, parentheses, quoted text, and a remembered value. Its output contains only the label and the value:

mean: 20.5

Python does not print the source line itself. It carries out the instruction. Keeping source and output separate prevents us from treating punctuation such as parentheses or quotation marks as part of the result.