CDT501 Chap.3 Variables, Types and Expressions
Variables, Types and Expressions
Make state changes visible
This chapter treats assignment as evaluation followed by rebinding. It traces values and types line by line, separates numeric arithmetic from string concatenation and repetition, and makes conversion a documented claim about valid input.
The closing method calculates a moderated project score inside the outer course weighting so that syntactically valid arithmetic cannot hide a mistaken assessment model.
What this chapter covers
- 01
Variable names and bindings
- 02
Assignment and augmented assignment
- 03
Numeric and string types
- 04
Explicit input conversion
- 05
Weighted calculations with intermediate values
Worked example · free
Calculate a two-level weighted result
- 3Compute the moderated group result.
- 3Apply the outer course weights.
- 2Explain why contribution is not separate.
Key terms
- Binding
- The current association between a name and a value or object.
- Expression
- A combination of values and operations that Python evaluates to produce a result.
- Integer
- A whole-number value supporting arithmetic without a fractional part.
- Float
- A numerical value represented with a decimal or floating-point form.
- Conversion
- An explicit operation creating a value of a requested type from another representation.
Variables, Types and Expressions FAQ
Why is x equals x plus one valid in Python?
Python reads it as assignment rather than a timeless mathematical equality. The right side uses the current value bound to x, adds one and produces a result. Only after that evaluation succeeds does the left side update the binding so x refers to the new value.
Why can multiplying text by a number produce text?
String multiplication means repetition, so a text value such as four repeated three times becomes three copies of the characters. Python follows the operand types rather than guessing that the text represents a quantity. Validate and convert the input before arithmetic when numerical meaning is intended.
How can I expose a wrong weighting formula?
Use unequal component values and make the teacher and contribution scores noticeably different. Equal values allow several incorrect formulas to return the same result. Print the moderated group value separately, verify that the outer weights total one, and compare the final number with a hand calculation.
Why should conversion happen at the input boundary?
Converting near the point of input gives the rest of the program a clear promise about type and validity. First preserve the raw value long enough to report a useful error, then check the permitted form and convert it once. If conversion is scattered across later expressions, different paths may interpret the same text differently, and an error can appear far from its cause.
Boundary conversion also makes tests sharper: one set checks rejected inputs, while the calculation tests can use already validated numbers. Do not convert merely because an operator fails; decide whether decimals, signs, blank text and surrounding spaces belong to the accepted domain, document that decision, and test each boundary explicitly.
Assessment move
Build a state table with one row per executed line and columns for value and type. Expand augmented assignment into its read, calculate and rebind meaning. For every input conversion, list one valid and two invalid strings. Recalculate the course-weight example with new unequal scores and verify the inner project formula before the outer weighted sum. Start each trace by copying only names that currently exist.
Evaluate the right-hand expression from that before-state, then update the binding on the left. Never revise an earlier row after a later reassignment; the whole point is to preserve sequence. Add separate columns for printed output and returned value so a visible number is not mistaken for stored state. Practise one expression with two integers, two strings and a mixed pair.
Predict addition, concatenation or an exception from the operand types before calculating. Repeat with multiplication to separate numerical scaling from string repetition. For division, use one even and one non-even case so ordinary and floor division cannot accidentally agree. At every input boundary, write the original text, validation rule and converted type.
Test empty text, decimal text and a word as well as a valid integer. Treat a failed conversion as information about the data rather than replacing it silently. For weighted results, select unequal scores and make the contribution score far from the teacher score. Compute the moderated group value as a named intermediate, check the outer weights sum to one, then compare the final result with an independent hand calculation.
Finally change only one component and predict the direction and size of the result change. This sensitivity check reveals whether the intended weight actually reaches the formula and whether a nested contribution has been counted twice.
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