How Python executes code - Visual Walkthrough

Concept Flow - How Python executes code

Start: Python reads code

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Tokenize: split code into pieces

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Parse: build structure (syntax tree)

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Compile: convert to bytecode

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Execute: run bytecode step-by-step

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Output results or errors

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End

Python reads your code, breaks it into parts, understands the structure, turns it into instructions, runs them, and shows results.

Execution Sample

Python

x = 5
print(x + 3)

Assign 5 to x, then print x plus 3.

Execution Table

Step	Action	Code Line	Variable State	Output
1	Read and tokenize	x = 5	{}
2	Parse and compile	x = 5	{}
3	Execute assignment	x = 5	{"x": 5}
4	Read and tokenize	print(x + 3)	{"x": 5}
5	Parse and compile	print(x + 3)	{"x": 5}
6	Execute print	print(x + 3)	{"x": 5}	8
7	End of code		{"x": 5}

💡 Reached end of code, all instructions executed.

Variable Tracker

Variable	Start	After Step 3	After Step 6	Final
x	undefined	5	5	5

Key Moments - 3 Insights

Why does Python first read and tokenize the code before running it?

Python needs to break the code into small pieces (tokens) to understand what each part means before it can run it, as shown in steps 1 and 4 of the execution table.

What happens when Python executes the assignment 'x = 5'?

Python stores the value 5 in the variable x, updating the variable state as seen after step 3 in the execution table.

Why does the print statement output 8?

Because Python calculates x + 3 using the current value of x (5), resulting in 8, which is printed at step 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the variable state after step 3?

A {}

B {"x": 5}

C {"x": 8}

D undefined

💡 Hint

Check the 'Variable State' column at step 3 in the execution table.

At which step does Python output the number 8?

A Step 3

B Step 4

C Step 6

D Step 7

💡 Hint

Look at the 'Output' column in the execution table to find when 8 appears.

If the code was changed to 'x = 10' instead of 'x = 5', what would be the output at step 6?

A 13

B 8

C 5

D 10

💡 Hint

Refer to the variable_tracker and execution_table to see how x affects the output.

Concept Snapshot

Python execution steps:
1. Read and tokenize code
2. Parse into structure
3. Compile to bytecode
4. Execute instructions
Variables update as code runs
Output shows results or errors

Full Transcript

Python runs code by first reading it and breaking it into small pieces called tokens. Then it understands the structure by parsing. Next, it compiles the code into bytecode, which is a set of instructions it can run. Finally, Python executes these instructions step-by-step, updating variables and showing output. For example, when running 'x = 5' and 'print(x + 3)', Python stores 5 in x and then prints 8 because it adds 3 to x's value.