Converting to Strings with str()

One Value, Three Representations

A value can be converted into different forms depending on what a calculation or display needs. float() produces a floating-point number, int() produces an integer, and str() represents a value as text. The conversion function does not merely rename the original value: it determines how the result is represented and what information is retained.

Following a Value Through Conversion

Start with the value 7. Converting it with float() gives a floating-point representation, so the result has decimal notation even though the original value did not. Converting it with str() changes the value into text. Converting it with int() keeps it as an integer. These operations answer different questions: should the result support fractional numeric calculations, remain an integer, or be treated as text?

What Each Function Preserves

float() is used when a value needs to be in floating-point form. It converts integers and numeric strings to floating-point numbers. For an integer such as 7, float() adds decimal representation where none existed. For a numeric string such as 7.25, float() converts the text into a floating-point number and preserves its fractional value.

int() has a different behavior. When it receives a floating-point value, it truncates the decimal portion. Truncation means chopping off everything after the decimal point; it does not mean choosing the nearest integer. Therefore, 3.99999 becomes 3, not 4. The same rule applies regardless of how large the decimal portion is.

str() is the conversion to use when the result should be text rather than a numeric value. This makes it different from int() and float(): those functions produce numeric forms, while str() represents the value as a string.

Decimal Portions After int()

Acceptable Inputs and Errors

int() works with floating-point values and whole-number strings. It raises an error for decimal strings and for invalid input. For example, a string containing a decimal representation cannot be passed directly to int() according to the source behavior described here. To convert a decimal string such as '3.14' to an integer, first convert the string to a floating-point number and then convert that result to an integer: int(float('3.14')). The intermediate float conversion handles the decimal string; the later int conversion truncates the decimal portion.

float() accepts integers and numeric strings. If the input string does not represent a valid number, float() raises a ValueError. This distinction matters when processing user input: text that looks like a number can be converted, but nonnumeric text cannot be treated as a floating-point number by float().

Mistakes in Conversion Choices

• Expecting int() to round a decimal value.

  int() truncates by removing the decimal portion; it does not round to the nearest integer.

  Fix: Expect the result 3.

• Passing a decimal string directly to int().

  int() works with whole-number strings, but a decimal string raises an error.

  Fix: Convert the string to float first, then convert that result to int.

• Treating str() as a numeric conversion.

  str() produces text, whereas int() and float() produce numeric forms.

  Fix: Use int() for an integer result or float() for a floating-point result.

• Assuming float() accepts every string.

  float() raises a ValueError for a string that is not a valid number.

  Fix: Check that the input string represents a number before relying on float() conversion.

Choosing the Conversion

Choose the function based on the form you need after conversion. Use float() when fractional values must be preserved or when an integer needs floating-point form. Use int() when an integer result is required and truncating any decimal portion is acceptable. Use str() when the value must be represented as text. When converting user input, remember that input commonly arrives as strings, so numeric calculations require an appropriate numeric conversion rather than str().

Check Your Prediction

Key Takeaways

1. float() converts integers and numeric strings to floating-point numbers and preserves fractional values.
2. int() truncates decimal portions; it does not round them.
3. int() accepts floats and whole-number strings but raises an error for decimal strings or invalid input.
4. float() raises a ValueError when a string does not represent a valid number.
5. str() represents a value as text, unlike int() and float(), which produce numeric forms.