Chapter 030 / 132 · Level 2 · Core Python

Inputs outputs and validation

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Chapter 30 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

Input is always text until converted. Verify the kind of text you received before applying numeric operations. Real applications must also consider negative numbers and sensible ranges.

Learning goal: Inputs outputs and validation. 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:

print and input
Conversion with int and float
Preventing crashes from invalid values
Clear user-facing feedback

03 · Pinpoint: read the code line by line

Line 1raw_age = '21'Create, update, or use a value. Notice what values enter this statement and which names it affects.
Line 2if raw_age.isdecimal():Control the next step. Notice what values enter this statement and which names it affects.
Line 3age = int(raw_age)Create, update, or use a value. Notice what values enter this statement and which names it affects.
Line 4print('Next year:', age + 1)Display a result. Notice what values enter this statement and which names it affects.
Line 5else:Control the next step. Notice what values enter this statement and which names it affects.
Line 6print('Please enter a whole number')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

Inputs outputs and validation: working demonstration

Python · runnable

PYTHON CODE

raw_age = '21'
if raw_age.isdecimal():
    age = int(raw_age)
    print('Next year:', age + 1)
else:
    print('Please enter a whole number')

Verified execution ▼

Next year: 22

Why it works: Input is always text until converted. Verify the kind of text you received before applying numeric operations. Real applications must also consider negative numbers and sensible ranges.

Worked example 2 · Inputs Outputs And Validation in practice

Python · executed and checked

PYTHON CODE

raw = '  42  '
text = raw.strip()
print('Valid:', text.isdigit())
print('Number:', int(text))

Verified execution output

Valid: True
Number: 42

Pinpoint learning: This practical inputs outputs and validation 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: raw = ' 42 ' — Store or update the value on the left using the expression on the right.
2: text = raw.strip() — Store or update the value on the left using the expression on the right.
3: print('Valid:', text.isdigit()) — Display the value so you can compare it with the verified output.
4: print('Number:', int(text)) — Display the value so you can compare it with the verified output.

05 · Independent practice and answer guidance

Exercise A · Adapt the code

Ask the user for a number with input(), recover safely if the text is invalid.

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

Input is always text until converted. Verify the kind of text you received before applying numeric operations. Real applications must also consider negative numbers and sensible ranges.

Next year: 22

06 · Build, verify and advance

Real-world task: Build an interactive age-and-date questionnaire with validation.

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.