0017-type-conversion-in-python.md

Type Conversion in Python

Type conversion (also called type casting) is the process of converting one data type into another.
Python supports explicit type conversion using built-in functions like:

• int()
• float()
• str()
• bool()
• list()
• tuple()
• set()
• dict()
• complex()

x = "10"
y = int(x)  # string → int

Type conversion is commonly used when:

• Taking user input
• Parsing data
• Performing calculations
• Cleaning or transforming data

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Key Rules of Type Conversion

Rule	Explanation
Explicit	Python does NOT auto-convert between most types
Data must be compatible	Invalid formats raise errors
Some data is lost	Example: int(3.9) → 3
Containers behave differently	Dict → list gives keys only

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1. Integer (int) Conversion

From Float to Int (Decimal Truncated)

float_val = 3.14
int_val = int(float_val)
print(int_val)

Output:

3

Decimal part is discarded, not rounded.

---

From String to Int (Numeric Strings Only)

str_val = "42"
int_val = int(str_val)
print(int_val)

Output:

42

Invalid string:

# int("abc")

Error:

ValueError: invalid literal for int()

---

From Boolean to Int

print(int(True))
print(int(False))

Output:

1
0

---

2. Float (float) Conversion

From Int to Float

int_val = 10
float_val = float(int_val)
print(float_val)

Output:

10.0

---

From String to Float

str_val = "3.14"
float_val = float(str_val)
print(float_val)

Output:

3.14

Invalid string:

# float("abc")

Error:

ValueError: could not convert string to float

---

From Boolean to Float

print(float(True))
print(float(False))

Output:

1.0
0.0

---

3. String (str) Conversion

From Int, Float, Boolean

print(str(123))
print(str(4.56))
print(str(True))

Output:

123
4.56
True

---

From List to String

list_val = [1, 2, 3]
print(str(list_val))

Output:

[1, 2, 3]

This does not join elements — it creates a textual representation.

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4. Boolean (bool) Conversion

Truthy vs Falsy Values

Value	Result
0, 0.0	False
Empty string ""	False
Empty list []	False
Empty tuple ()	False
Empty dict {}	False
Everything else	True

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Examples

print(bool(0))
print(bool(10))
print(bool(""))
print(bool("hello"))
print(bool([]))
print(bool([1, 2, 3]))

Output:

False
True
False
True
False
True

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5. List (list) Conversion

From String

str_val = "Hello"
list_val = list(str_val)
print(list_val)

Output:

['H', 'e', 'l', 'l', 'o']

---

From Tuple

tuple_val = (1, 2, 3)
print(list(tuple_val))

Output:

[1, 2, 3]

---

From Set

set_val = {4, 5, 6}
print(list(set_val))

Output (order may vary):

[4, 5, 6]

---

From Dictionary (Keys Only)

dict_val = {'a': 1, 'b': 2}
print(list(dict_val))

Output:

['a', 'b']

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6. Tuple (tuple) Conversion

From String

str_val = "Python"
print(tuple(str_val))

Output:

('P', 'y', 't', 'h', 'o', 'n')

---

From List / Set / Dict

print(tuple([7, 8, 9]))
print(tuple({10, 11, 12}))
print(tuple({'x': 1, 'y': 2}))

Output (order may vary):

(7, 8, 9)
(10, 11, 12)
('x', 'y')

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7. Set (set) Conversion

From String (Duplicates Removed)

str_val = "banana"
print(set(str_val))

Output:

{'b', 'a', 'n'}

---

From List / Tuple / Dict

print(set([1, 2, 2, 3]))
print(set((4, 5, 5, 6)))
print(set({'apple': 1, 'orange': 2}))

Output:

{1, 2, 3}
{4, 5, 6}
{'apple', 'orange'}

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8. Dictionary (dict) Conversion

From List of Tuples

list_of_tuples = [('a', 1), ('b', 2)]
print(dict(list_of_tuples))

Output:

{'a': 1, 'b': 2}

---

From List of Lists

list_of_lists = [['x', 10], ['y', 20]]
print(dict(list_of_lists))

Output:

{'x': 10, 'y': 20}

---

Invalid conversion:

# dict([1, 2, 3])

Error:

ValueError: dictionary update sequence element #0 has length 1; 2 is required

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9. Complex (complex) Conversion

From Int and Float

print(complex(3))
print(complex(2.5))

Output:

(3+0j)
(2.5+0j)

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From Real and Imaginary Parts

real = 4
imaginary = 5
print(complex(real, imaginary))

Output:

(4+5j)

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Common Errors

Error	Cause	Example
ValueError	Invalid numeric string	int("abc")
TypeError	Unsupported conversion	dict([1,2,3])
Data Loss	Truncation	int(3.9) → 3
Order Loss	Set conversions	set([3,1,2])

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Summary

Conversion	Notes
int()	Truncates floats
float()	Supports numeric strings
str()	Converts anything to text
bool()	Uses truthy/falsy rules
list()	Breaks iterable into elements
tuple()	Immutable version of list
set()	Removes duplicates
dict()	Needs key-value pairs
complex()	Used for complex numbers

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Practice Tasks

1. Convert user input "25" into an integer and add 10.
2. Convert [1, 2, 2, 3] into a set, then back into a list.
3. Convert "Python" into a tuple and reverse it.
4. Try converting "3.14" to int — observe the error.
5. Create a dictionary from [("a",1), ("b",2)] and access value "b".