How variable type changes at runtime in Python - Quick Revision & Summary

Recall & Review

beginner

What does it mean that Python variables can change type at runtime?

It means a variable can hold a value of one type at one moment, and later hold a value of a different type without errors.

beginner

Show an example where a variable changes from an integer to a string.

x = 5 # x is an integer x = "hello" # now x is a string

intermediate

Why can Python variables change type at runtime?

Because Python uses dynamic typing, it does not fix variable types when you write code. Types are checked when the program runs.

beginner

What happens if you try to use a variable in a way that doesn't match its current type?

Python will raise an error if the operation is not supported for the variable's current type.

beginner

Can you assign a list to a variable that was previously an integer? Why?

Yes, because Python variables are just names pointing to values. You can assign any type of value to the same variable name.

What type does a Python variable have?

A A fixed type set when the variable is created

B Always string

C The type of the value it currently holds

D Always integer

Python variables have the type of the value they currently hold, not a fixed type.

What happens if you assign a string to a variable that was holding an integer?

A The variable changes to hold the string

B Python gives a syntax error

C The variable keeps the integer value

D The program crashes immediately

Python allows variables to change type by assigning a new value of a different type.

Which typing system allows variables to change type at runtime?

A Static typing

B Dynamic typing

C Strong typing

D Weak typing

Dynamic typing means types are checked at runtime and variables can change type.

If x = 10, then x = 'ten', what is the type of x now?

A boolean

B integer

C float

D string

After assigning 'ten', x holds a string value.

What error occurs if you try to add a number and a string in Python?

A TypeError

B SyntaxError

C NameError

D ValueError

Python raises a TypeError when you try to perform unsupported operations between incompatible types.

Explain how Python variables can change their type during program execution.

Think about how Python treats variable names and values.

Give an example where a variable changes from one type to another and explain what happens.

Use simple types like integer and string.

Practice

1. What happens to the type of a Python variable when you assign a new value of a different type to it?

easy

A. The variable's type changes to the new value's type automatically.

B. The variable keeps its original type regardless of the new value.

C. Python raises an error if the new value's type is different.

D. The variable becomes a list containing both old and new types.

Solution

1. Step 1: Understand Python variable typing

   Python variables are dynamically typed, meaning their type depends on the current value assigned.

2. Step 2: Assigning a new value changes the type

   When you assign a new value of a different type, the variable's type updates to match the new value automatically.

3. Final Answer:

   The variable's type changes to the new value's type automatically. -> Option A

4. Quick Check:

   Python variables are dynamic = A [OK]

Hint: Remember: Python variables follow the value's type, not fixed type. [OK]

Common Mistakes:

• Thinking variables have fixed types like in some other languages.
• Assuming Python throws an error on type change.
• Believing variable types combine old and new types.

2. Which of the following is the correct way to assign a new value to a variable in Python so its type changes?

easy

A. x = 10

B. x := 10

C. int x = 10

D. var x = 10

Solution

1. Step 1: Identify Python assignment syntax

   Python uses the single equals sign (=) to assign values to variables.

2. Step 2: Check each option

   x = 10 uses correct syntax. x := 10 uses walrus operator which is for expressions, not simple assignment. int x = 10 and var x = 10 are invalid in Python.

3. Final Answer:

   x = 10 -> Option A

4. Quick Check:

   Assignment uses = in Python = A [OK]

Hint: Use single '=' for assignment in Python, not ':=' or types. [OK]

Common Mistakes:

• Confusing ':=' walrus operator with assignment.
• Using typed declarations like other languages.
• Trying to declare variable types explicitly.

3. What will be the output of this code?

var = 5
print(type(var))
var = 'hello'
print(type(var))

medium

A. \n

B. \n

C. \n

D. Error

Solution

1. Step 1: Check initial assignment and type

   var is first assigned 5, an integer, so type(var) is <class 'int'>.

2. Step 2: Reassign and check new type

   var is then assigned 'hello', a string, so type(var) is <class 'str'>.

3. Final Answer:

   <class 'int'>\n<class 'str'> -> Option C

4. Quick Check:

   Type changes with value = C [OK]

Hint: Track variable type after each assignment line by line. [OK]

Common Mistakes:

• Assuming type stays the same after reassignment.
• Confusing output order of print statements.
• Expecting a runtime error on type change.

4. Find the error in this code snippet:

value = 10
value = value + '5'
print(value)

medium

A. NameError because 'value' is undefined

B. SyntaxError due to invalid assignment

C. No error, output is 15

D. TypeError because int and str cannot be added

Solution

1. Step 1: Analyze the operation between int and str

   value is an int (10), then tries to add a string ('5'), which is not allowed.

2. Step 2: Identify the error type

   Python raises a TypeError when adding incompatible types like int and str.

3. Final Answer:

   TypeError because int and str cannot be added -> Option D

4. Quick Check:

   Adding int + str causes TypeError = D [OK]

Hint: Check types before adding; int + str causes error. [OK]

Common Mistakes:

• Thinking Python auto-converts types in addition.
• Expecting output '15' as string concatenation.
• Confusing SyntaxError with TypeError.

5. Given the code below, what will be the final type of data?

data = 100
if isinstance(data, int):
    data = str(data)
else:
    data = [data]

hard

A. int

B. str

C. list

D. bool

Solution

1. Step 1: Check initial type of data

   data starts as 100, which is an int.

2. Step 2: Evaluate the if condition

   Since data is int, the if block runs, converting data to str('100').

3. Final Answer:

   str -> Option B

4. Quick Check:

   int converted to str by condition = B [OK]

Hint: If int, convert to str; else make list. [OK]

Common Mistakes:

• Ignoring the if condition and assuming list.
• Thinking data remains int after reassignment.
• Confusing str and bool types.