Chapter 034 / 132 · Level 2 · Core Python

For loops and iteration

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Chapter 34 of 132 · Level 2 · Core Python · Python demonstrations with recorded outputs. Learn through reading, predicting and testing real Python code.

Code + real outputPractice + explanationsOffline Python exercises

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01 · Why this concept matters

for walks over values from an iterable. enumerate provides both an item and its position without creating a manual counter.

Learning goal: For loops and iteration. Start by reading what the program does, then inspect its exact output. Where an example illustrates only a prerequisite, study the named advanced feature separately before using it in production.

02 · The important subtopics

Study these connected ideas as you work through the demonstration:

for with sequences and ranges
enumerate and zip
Loop nesting
Common off-by-one errors

03 · Pinpoint: read the code line by line

Line 1names = ['Mina', 'Omid', 'Sara']Create, update, or use a value. Notice what values enter this statement and which names it affects.
Line 2for index, name in enumerate(names, 1):Control the next step. Notice what values enter this statement and which names it affects.
Line 3print(index, name)Display a result. Notice what values enter this statement and which names it affects.

Input → operation → output

Find the literal input values and arguments, follow each operation in execution order, and verify the result below. This habit turns a code listing into a testable explanation.

04 · Runnable Python example and verified output

For loops and iteration: working demonstration

Python · runnable

PYTHON CODE

names = ['Mina', 'Omid', 'Sara']
for index, name in enumerate(names, 1):
    print(index, name)

Verified execution ▼

1 Mina
2 Omid
3 Sara

Why it works: for walks over values from an iterable. enumerate provides both an item and its position without creating a manual counter.

Worked example 2 · For Loops And Iteration in practice

Python · executed and checked

PYTHON CODE

names = ['Ali', 'Bea', 'Cora']
for index, name in enumerate(names, 1):
    print(f'{index}. {name}')

Verified execution output

1. Ali
2. Bea
3. Cora

Pinpoint learning: This practical for loops and iteration demonstration uses a small input you can inspect. Run it exactly as shown, compare the displayed result, then change one value and predict how the result will change.

Understand the code line by line

1: names = ['Ali', 'Bea', 'Cora'] — Store or update the value on the left using the expression on the right.
2: for index, name in enumerate(names, 1): — Repeat the indented block for each item or while the condition remains true.
3: print(f'{index}. {name}') — Display the value so you can compare it with the verified output.

Worked example 3 · Change one input and compare

Python · executed and checked

PYTHON CODE

names = ['Ali!', 'Bea', 'Cora']
for index, name in enumerate(names, 1):
    print(f'{index}. {name}')

Verified execution output

1. Ali!
2. Bea
3. Cora

Pinpoint learning: Exactly one literal input was changed from the preceding demonstration. Compare the outputs: identify the modified value, trace the operation, and explain which result changed. The displayed output was produced by executing this variant.

Understand the code line by line

1: names = ['Ali!', 'Bea', 'Cora'] — Store or update the value on the left using the expression on the right.
2: for index, name in enumerate(names, 1): — Repeat the indented block for each item or while the condition remains true.
3: print(f'{index}. {name}') — Display the value so you can compare it with the verified output.

05 · Independent practice and answer guidance

Exercise A · Adapt the code

Use zip to print pairs of subjects and scores.

Exercise B · Predict

Before pressing Run, predict the first displayed line. Next, change one literal value and record the new observed output. Explain the difference in one clear sentence.

Exercise C · Debug

Make a temporary invalid change, read the exception or failed assertion if one occurs, restore the working code, and explain the repair.

Worked explanation for the original example

for walks over values from an iterable. enumerate provides both an item and its position without creating a manual counter.

1 Mina
2 Omid
3 Sara

06 · Build, verify and advance

Real-world task: Generate multiplication tables and a weekly schedule.

Mastery checklist

Run the original code; explain the important lines; change one input and verify the new outcome; use a test or recorded result to justify your conclusion.