Kurage Notes

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String Representation ​

String Representation of Objects ​

An object value should behave like the kind of data it is meant to represent. For instance, by producing a string representation of itself.

There are two main ways to produce the "string" of an object in Python: str() and repr().

  • str() is used to describe the object to the end user in a "Human-readable" form
  • repr() can be thought of as a "Computer-readable" form mainly used for debugging and development.

repr ​

The repr function returns a Python expression (a string) that evaluates to an equal object.

python
# For most object types
eval(repr(object)) == object

The result of calling repr on a value is what Python prints in an interactive session.

python
>>> 12e12
12000000000000.0
>>> repr(12e12)
'12000000000000.0'
>>> print(repr(12e12))
12000000000000.0
python
>>> s = "Hello, World!"
>>> repr(s)
"'Hello, World!'"
>>> str(s)
'Hello, World!'
>>> print(str(s))
Hello, World!
>>> print(repr(s))
'Hello, World!'

Some objects do not have a simple Python-readable string

str ​

The str function returns a human-readable string interpreting the input value. The result of calling str on the value of an expression is what Python prints using the print function.

python
>>> from fractions import Fraction
>>> half = Fraction(1, 2)
>>> str(half)
'1/2'
>>> print(half)
1/2

String Interpolation ​

F-Strings ​

String interpolation involves evaluating a string literal that contains expressions.

python
>>> from math import pi
>>> f'pi starts with {pi}...'
'pi starts with 3.14159...'
>>> print(f'pi starts with {pi}...')
pi starts with 3.14159...

The result of evaluating an f-string literal contains the str (stir) string of the value of each sub-expression.

python
>>> from fractions import Fraction
>>> half = Fraction(1, 2)
>>> str(half)
1/2
>>> f'half of a half is {half * half}'
'half of a half is 1/4'

Sub-expressions are evaluated in the current enviroment. When a sub-expression is evaluated, it behaves just as normal, which means that there might be side effects.

Formatting Strings ​

python
>>> 'I have {0} {1}'.format(2, "parameters")
'I have 2 parameters'