How to convert an object x to an expression string in Python?

In Python, an expression is any statement that evaluates to a value when executed. Converting an object to an expression string means creating a string representation that can be evaluated back to recreate the original object.

What's an Expression?

Python expressions are code snippets that return a value when evaluated ?

print(4 + 6)
print(22172 * 27282)
print(max(12, 9330))

10
604896504
9330

We can evaluate a valid Python expression using the eval() function to convert strings back to objects.

Converting Objects to Expression Strings

Python provides several methods to convert objects to expression strings ?

• Using the repr() function

• Using the str() function

• Creating custom __repr__() methods

Using repr() Function

The repr() function returns an unambiguous string representation that ideally can be evaluated with eval() to recreate the object ?

x = [3, 2, 1]
expr_str = repr(x)
print("Expression string:", expr_str)
print("Type:", type(expr_str))

# Recreate object from expression string
x_copy = eval(expr_str)
print("Recreated object:", x_copy)
print("Objects equal:", x == x_copy)

Expression string: [3, 2, 1]
Type: <class 'str'>
Recreated object: [3, 2, 1]
Objects equal: True

With Dictionary Objects

data = {'a': 2, 'b': 4}
expr_str = repr(data)
print("Expression string:", expr_str)

# Recreate from string
data_copy = eval(expr_str)
print("Recreated object:", data_copy)

Expression string: {'a': 2, 'b': 4}
Recreated object: {'a': 2, 'b': 4}

Using str() Function

The str() function returns a human-readable string representation, but may not always be evaluable ?

import datetime

today = datetime.datetime.now()
print("str() output:", str(today))
print("repr() output:", repr(today))

str() output: 2024-01-15 10:30:45.123456
repr() output: datetime.datetime(2024, 1, 15, 10, 30, 45, 123456)

Custom repr() for Objects

Define __repr__() method to control how objects convert to expression strings ?

class Point:
    def __init__(self, a, b):
        self.a = a
        self.b = b
    
    def __repr__(self):
        return f"Point({self.a}, {self.b})"
    
    def __str__(self):
        return f"({self.a}, {self.b})"

p = Point(4, 5)
print("repr():", repr(p))
print("str():", str(p))

# Can recreate object from repr() output
expr_str = repr(p)
print("Expression string:", expr_str)

repr(): Point(4, 5)
str(): (4, 5)
Expression string: Point(4, 5)

Comparison

Conclusion

Use repr() to create evaluable expression strings for debugging and object recreation. Use str() for human-readable display. Implement custom __repr__() methods for meaningful object representations.