Logical operators in conditions in Python - Step-by-Step Execution

Concept Flow - Logical operators in conditions

Start

↓

Evaluate Condition 1

↓

Evaluate Condition 2

↓

Apply Logical Operator

↓

Result: True or False

↓

If True -> Execute Block

↓

If False -> Skip Block

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End

The program checks each condition, combines them with logical operators (and, or, not), then decides which code to run based on the combined result.

Execution Sample

Python

x = 5
if x > 0 and x < 10:
    print("x is between 1 and 9")

This code checks if x is greater than 0 AND less than 10, then prints a message if true.

Execution Table

Step	Expression Evaluated	Value	Logical Operator	Combined Result	Branch Taken	Output
1	x > 0	5 > 0 is True	and
2	x < 10	5 < 10 is True	and	True and True is True	If True block	x is between 1 and 9
3	End				Exit

💡 All conditions evaluated; combined result True; executed print statement; program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	5	5	5	5

Key Moments - 3 Insights

Why do both conditions need to be true for the 'and' operator to run the print?

Because 'and' requires all conditions to be true to return True. In the execution_table rows 1 and 2, both conditions are True, so the combined result is True and the print runs.

What happens if the first condition is False with 'and' operator?

The combined result becomes False immediately, so the second condition is not checked and the print block is skipped. This is called short-circuit evaluation.

How does 'or' differ from 'and' in conditions?

'Or' returns True if at least one condition is True. It stops checking further once it finds a True condition (short-circuit). 'And' needs all True.

Visual Quiz - 3 Questions

Look at the execution_table, what is the combined result after evaluating both conditions?

A False

B True

C None

D Error

💡 Hint

Check the 'Combined Result' column in row 2 of the execution_table.

At which step does the program decide to execute the print statement?

A Step 2

B Step 3

C Step 1

D Before Step 1

💡 Hint

Look at the 'Branch Taken' and 'Output' columns in the execution_table.

If x was 15, what would happen to the combined result and output?

A Error occurs

B Combined result True, print runs

C Combined result False, no print

D Combined result True, no print

💡 Hint

Think about how 'x < 10' would evaluate if x=15, referencing the conditions in the execution_table.

Concept Snapshot

Logical operators combine conditions:
- and: True if all True
- or: True if any True
- not: reverses True/False
Use in if statements to control flow based on multiple checks.

Full Transcript

This lesson shows how Python uses logical operators like 'and' to combine conditions in if statements. The program checks each condition, then combines their True/False results. If the combined result is True, it runs the code inside the if block. For example, if x is 5, the conditions 'x > 0' and 'x < 10' are both True, so the print statement runs. If any condition was False with 'and', the print would not run. This is called short-circuit evaluation. Understanding this helps control program decisions based on multiple rules.

Practice

1. Which logical operator in Python means all conditions must be true for the whole expression to be true?

easy

A. xor

B. and

C. not

D. or

Solution

1. Step 1: Understand the meaning of 'and'

   The and operator returns true only if both conditions are true.

2. Step 2: Compare with other operators

   or needs only one true condition, not reverses truth, and xor is not a Python keyword.

3. Final Answer:

   and -> Option B

4. Quick Check:

   All conditions true = and [OK]

Hint: Remember: 'and' means all must be true [OK]

Common Mistakes:

• Confusing 'and' with 'or'
• Thinking 'not' means all true
• Using 'xor' which is not a Python keyword

2. Which of the following is the correct syntax to check if variable x is NOT equal to 10 and variable y is greater than 5?

easy

A. if x != 10 and y > 5:

B. if x <> 10 and y > 5:

C. if x != 10 & y > 5:

D. if x != 10 or y > 5:

Solution

1. Step 1: Check the inequality operator

   != is the correct 'not equal' operator in Python; <> is invalid syntax.

2. Step 2: Check logical operator usage

   and correctly combines two conditions; & is a bitwise operator and or changes logic.

3. Final Answer:

   if x != 10 and y > 5: -> Option A

4. Quick Check:

   Correct inequality and 'and' syntax [OK]

Hint: Use '!=' for not equal and 'and' to combine conditions [OK]

Common Mistakes:

• Using '<>' instead of '!='
• Using '&' instead of 'and'
• Using 'or' when 'and' is needed

3. What will be the output of this code?

age = 20
has_id = False
if age >= 18 and has_id:
    print('Allowed')
else:
    print('Denied')

medium

A. SyntaxError

B. Allowed

C. Denied

D. No output

Solution

1. Step 1: Evaluate each condition

   age >= 18 is True since 20 >= 18, but has_id is False.

2. Step 2: Apply 'and' operator

   True and False is False, so the else block runs.

3. Final Answer:

   Denied -> Option C

4. Quick Check:

   True and False = False [OK]

Hint: Both conditions must be true for 'and' to print Allowed [OK]

Common Mistakes:

• Assuming one True is enough with 'and'
• Confusing 'and' with 'or'
• Ignoring boolean value of variables

4. Find the error in this code snippet:

if not x > 10 or y < 5
    print('Check passed')

medium

A. Missing colon ':' after condition

B. Wrong use of 'not' operator

C. Incorrect indentation

D. Using 'or' instead of 'and'

Solution

1. Step 1: Check syntax of if statement

   Python requires a colon ':' at the end of the if condition line.

2. Step 2: Review other parts

   The use of not and or is correct; indentation is not shown as wrong here.

3. Final Answer:

   Missing colon ':' after condition -> Option A

4. Quick Check:

   if statements need ':' [OK]

Hint: Always put ':' after if condition [OK]

Common Mistakes:

• Forgetting ':' after if
• Misusing 'not' operator
• Wrong indentation without colon

5. You want to check if a number n is either less than 0 or greater than 100, but NOT equal to -10. Which condition correctly uses logical operators?

hard

A. if n < 0 or (n > 100 and n != -10):

B. if n < 0 and n > 100 or n != -10:

C. if not (n < 0 or n > 100) and n == -10:

D. if (n < 0 or n > 100) and n != -10:

Solution

1. Step 1: Understand the condition requirements

   We want numbers less than 0 or greater than 100, but exclude -10.

2. Step 2: Analyze each option

   if (n < 0 or n > 100) and n != -10: correctly groups the or condition and excludes -10 with and n != -10. if n < 0 or (n > 100 and n != -10): excludes -10 only when >100, not <0 (e.g., n=-10 is true). if not (n < 0 or n > 100) and n == -10: reverses logic and checks for equal -10. if n < 0 and n > 100 or n != -10: mixes and/or incorrectly.

3. Final Answer:

   if (n < 0 or n > 100) and n != -10: -> Option D

4. Quick Check:

   Group or, then exclude -10 with and [OK]

Hint: Group or conditions, then exclude with and [OK]

Common Mistakes:

• Wrong grouping of conditions
• Misplacing 'not' or 'and'
• Confusing '!=' with '=='