Assignment, Statement by Statement
Try it: Assignment, Statement by Statement
How an assignment statement evaluates its right side first (using the names' current values) and only then binds the name on the left: updating x = x + 1, augmented operators like += and //=, chained a = b = 0, simultaneous a, b = b, a + b versus one-at-a-time assignment, rebinding a name to a value of another type, the NameError for a name that was never assigned, and the SyntaxError for a name that breaks the naming rules.
How it works
- Before anything runs, Python compiles the whole program; a name that breaks the rules (starts with a digit, contains a space or a dash, or is a keyword) is a SyntaxError and no line runs at all.
- For each assignment, evaluate the complete right side first, looking every name up in the variable table as it is now.
- Then bind the name(s) on the left to that value: a new name is added to the table, an existing one is replaced (even by a value of a different type).
- x += 5 reads x, applies the operator (+, -, *, //, %, **) and binds the result back to x; a = b = 0 evaluates 0 once and binds a, then b.
- a, b = b, a + b builds the whole right-hand tuple from the OLD values before binding a and b; two separate statements would use the new a in the second line.
- Reading a name that is not in the table raises NameError and the program stops at that line.
Default run (10 steps): A 4-line program. Python first compiles all of it, then runs it top to bottom. The variable table on the right starts empty. … Program finished. Final table: x = 16.
Simplified: The programs are fixed templates: you choose the template, its numbers, the operator and the variable name, never the code. The lab's own small Python tokenizer and parser (written for the statements these programs use: assignments, print calls, if / elif / else, for, while, def and return) checks the text; nothing is executed as real Python. Every program the lab can show was compiled by CPython 3.12 and the lab reproduces its exception type, message, line, caret columns and the printed report. A small interpreter written for the same statements then runs them (integers of any size, the str/float results these templates produce, and print); each program was also run in CPython 3.12 under sys.settrace, and the lab reproduces the variables after every line, the output and the error.
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