How variable type changes at runtime in Python - Visual Walkthrough

Concept Flow - How variable type changes at runtime

Assign value to variable

↓

Check value type

↓

Variable holds value and type

↓

Reassign variable with new value

↓

Check new value type

↓

Variable updates to new type

This flow shows how a variable in Python can hold a value of one type, then be reassigned to a value of a different type, changing its type at runtime.

Execution Sample

Python

x = 10
print(type(x))
x = 'hello'
print(type(x))

Assign an integer to x, print its type, then assign a string to x and print its new type.

Execution Table

Step	Code Line	Variable x Value	Type of x	Output
1	x = 10	10	int
2	print(type(x))	10	int	<class 'int'>
3	x = 'hello'	'hello'	str
4	print(type(x))	'hello'	str	<class 'str'>
5	End	'hello'	str	Execution stops

💡 Code ends after printing the type of x as str.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
x	undefined	10 (int)	'hello' (str)	'hello' (str)

Key Moments - 2 Insights

Why does the type of x change after reassignment?

Because Python variables are labels for values, not fixed types. When x is assigned a new value, it points to that new value and its type changes accordingly, as shown in steps 1 and 3 of the execution_table.

Does the original integer value 10 stay linked to x after reassignment?

No, after step 3, x no longer refers to 10. It now refers to the string 'hello'. The integer 10 still exists in memory if used elsewhere, but x points to the new string.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the output?

A <class 'str'>

B <class 'int'>

C 10

D 'hello'

💡 Hint

Check the Output column at step 2 in the execution_table.

At which step does the variable x change its type from int to str?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Type of x column in the execution_table between steps 2 and 3.

If we assign x = 3.14 after step 4, what would be the type of x?

A float

B str

C int

D bool

💡 Hint

Recall that 3.14 is a decimal number, which in Python is a float type.

Concept Snapshot

Python variables can hold any type of value.
Assigning a new value changes the variable's type.
Variable is a label pointing to a value.
Type is checked at runtime, not fixed.
Use type() to see current type.

Full Transcript

This example shows how a Python variable can change its type during program execution. Initially, x is assigned the integer 10, so its type is int. When we print type(x), it shows <class 'int'>. Later, x is assigned the string 'hello', changing its type to str. Printing type(x) then shows <class 'str'>. This happens because Python variables are just labels for values, and can point to any type of value at any time. The execution table traces each step, showing the value and type of x, and the output printed. This helps beginners see how variable types are dynamic in Python.

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.