Read a Line of Python
Name the parts of report lines you already understand: literals, names, operators, expressions, function calls, arguments, and statements. Read each line from its inner values to its complete effect.
We have already read and changed several Python lines. A small set of terms now lets us explain their parts precisely.
Values Come from Expressions
An expression is code that produces a value. In this line,
mean = total / count
total / count is an expression. It contains two names, total and count,
and the division operator /. The values used by an operator are its
operands.
A value written directly in source code is a literal. The numbers 4 and
20.5 and the string "°C" are literals:
A name is not the value itself. Python evaluates a name by finding the value it currently refers to.
A Statement Is a Complete Instruction
An assignment is a statement: a complete instruction that changes what a name refers to. Read it from the inside outward:
- find the current values of
totalandcount; - apply
/to produce 20.5; - assign that result to
mean.
This order connects the parts of the source line to its effect.
Q1. Find the value-producing part
In distance = abs(reading - 20), which complete part produces the value that
is assigned to distance?
Select one choice, then check.
HintRead the right side
The whole expression to the right of = produces the new value.
SolutionThe function call produces the value
Python evaluates reading - 20, passes that result to abs(), and assigns
the returned value to distance.
Function Calls Use Arguments
print() and abs() are built-in functions. A function call asks a
function to perform its task. The values or expressions between its
parentheses are arguments.
Consider the final report line:
print(f"Mean: {mean:.1f} {unit}")
Read it in this order:
- find the values referred to by
meanandunit; - build the formatted string
Mean: 20.5 °C; - pass that string as the argument to
print(); print()displays the text.
The inner f-string produces a value. The complete print() call is the
instruction that displays it. To follow this line, evaluate the inner parts
before the outer call.
| Source part | Role in the report line |
|---|---|
mean, unit | names whose current values are read |
"Mean: ..." | an f-string literal that builds report text |
f"Mean: {mean:.1f} {unit}" | the argument supplied to print() |
print(...) | the function call and complete instruction |
Q2. Read from the inside outward
What happens first while Python runs print("total:", 18 + 21)?
Select one choice, then check.
HintStart with the inner expression
print() needs the result of 18 + 21 before it can display the arguments.
SolutionThe addition runs first
Python calculates 39, then calls print() with "total:" and 39. The
output is total: 39.
Q3. Name each role
In total = reading_1 + reading_2, which items are operands of +?
Select one choice, then check.
HintLook beside the plus sign
The two names immediately to the left and right of + supply its values.
SolutionThe reading names are operands
Python reads the values of reading_1 and reading_2, then + combines
those two operands.
Literals and names supply values; operators and function calls form expressions; a statement gives Python a complete instruction. Reading from the inside outward explains how a line produces its effect.