Type Conversion in Python
The important question is not whether one type is universally better. Choose the type that matches what the number represents: int for whole, discrete quantities and float for measurements, averages, and other values with fractional parts.
Two Ways to Represent Numbers
Python has two main numeric types: int for integers and float for floating-point numbers. Understanding the difference is the foundation for reasoning about numeric type conversion. An integer represents a whole number, while a float represents a number with a decimal part or a value written in floating-point notation.
The important question is not whether one type is universally better. Choose the type that matches what the number represents: int for whole, discrete quantities and float for measurements, averages, and other values with fractional parts.
Integer and Float Values
| Type | Represents | Typical uses | Example notation |
|---|---|---|---|
| int | Whole numbers | Counting items, indexing positions, discrete quantities | 2 |
| float | Numbers with decimal parts | Measuring distances, calculating averages, scientific data | 3.23 |
Python's two main numeric types serve different kinds of numerical data.
A number such as 2 is recognized as an integer. A number such as 3.23 is a float. The distinction is about the numeric type Python uses for the value, not merely about how many characters appear in the literal. Use int when the data is naturally whole and discrete. Use float when fractional values are part of the data.
Reading E Notation
Floats can be written in ordinary decimal notation, such as 3.23, or in scientific notation called E notation. In E notation, E or e means times 10 to the power of. The part before E supplies the starting number, and the signed number after E supplies the power of 10.
Expanding a negative exponent
Interpret the floating-point literal 52.3E-4.
- 1Read E
E means times 10 to the power of.
- 2Apply the exponent
The literal means 52.3 multiplied by 10 to the power of negative 4.
- 3Write the decimal form
52.3E-4 equals 52.3 × 10-4, which is 0.00523.
52.3E-4 and 0.00523 are two valid written forms of the same float value.
A negative exponent makes the resulting value smaller than the number before E. E notation is useful for very large or very small numbers, including examples such as 1.5E10 and 2.5E-6.
Converting the Numeric Representation
Type conversion changes which numeric type represents a value. The central distinction is that an integer representation is whole-number based, while a floating-point representation supports decimal values. When thinking about a conversion, track both the numerical quantity and its representation: ask whether the value remains the same and whether the type has changed.
Integers Without a Separate Long Type
Python's int type can represent integers of any size. Python does not use one type for ordinary integers and a separate long type for very large integers. A small integer and a very large integer are both handled with the same int type.
Do not carry the small-integer versus large-integer model from languages that use separate int and long types into Python. Python uses int for both.
Choosing the Appropriate Type
| Task | Appropriate type | Reason |
|---|---|---|
| Count items | int | A count is a whole, discrete quantity. |
| Index a position | int | An index represents a whole-number position. |
| Measure a distance | float | A measurement may contain a fractional part. |
| Calculate an average | float | An average may contain a decimal part. |
| Represent scientific data | float | Scientific values may be very large, very small, or fractional. |
Choose the numeric type that matches the meaning of the data.
For each task, decide whether int or float is the more appropriate type: counting books on a shelf, recording a measured distance, storing an index position, and representing a very small scientific value written with E notation.
Hints
- Ask whether the value is a whole, discrete quantity.
- A decimal measurement or scientific value points toward float.
What do you think happens?
Which type best matches each task: counting books, recording a measured distance, storing an index, and representing a very small scientific value?
Reveal answer
int, float, int, float
Counts and index positions are whole, discrete quantities. Measurements and scientific values can contain fractional parts or use floating-point notation.
Mistakes Beginners Make
Common mistake Assuming Python needs a separate type for very large integers.
What it looks likeThinking that a very large whole number must use long instead of int.
Why it failsPython's int type represents integers of any size, and Python has no separate long type.
Do this insteadUse the same int type for both ordinary and very large integers.
Common mistake Treating every numeric literal as an integer.
What it looks likeClassifying 3.23 or 52.3E-4 as int values.
Why it failsDecimal notation and E notation are forms of floating-point notation.
Do this insteadRecognize values such as 3.23 and 52.3E-4 as float values.
Common mistake Reading E as part of a variable name or ignoring the exponent.
What it looks likeReading 52.3E-4 without interpreting the -4 as a power of 10.
Why it failsE means times 10 to the power of, so the exponent changes the scale of the value.
Do this insteadExpand 52.3E-4 as 52.3 × 10^-4 = 0.00523.Common mistake Choosing a numeric type without considering what the data represents.
What it looks likeUsing float for a simple item count or int for a measurement that can contain a fractional part.
Why it failsThe type should match whether the quantity is whole and discrete or fractional and measurable.
Do this insteadChoose int for counts and indexes, and float for measurements, averages, and scientific data.
Key Takeaways
- 1Python has two main numeric types: int and float.
- 2Use int for whole numbers, counts, indexes, and other discrete quantities.
- 3Use float for decimal values, measurements, averages, and scientific data.
- 4E notation means times 10 to the power of; for example, 52.3E-4 equals 0.00523.
- 5Python's int type can represent integers of any size, so there is no separate long type.
At a glance
- Distinguish int values from float values by asking whether the data is whole and discrete or fractional and measurable.
- Interpret E notation as multiplication by a power of 10.
- Remember that Python uses one int type for integers of any size; it has no separate long type.
- Choose the numeric type according to the meaning and precision needs of the data.