Understanding When Python’s “a + b” Isn’t Just “a + b”

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💡 Problem Formulation: In Python, adding two variables together using the plus operator (+) is subject to the types of the operands. While it might seem straightforward to expect a + b to always be equivalent to a + b, type-specific behaviors can lead to unexpected results. This article breaks down scenarios where a + b may not behave as anticipated and provides ways to handle these cases effectively.

Method 1: Concatenation with Strings

When variables a and b are strings, the + operator performs concatenation, which can result in a + b that doesn’t resemble numeric addition. To ensure proper string formatting, it’s often necessary to explicitly convert numerical variables to strings before concatenation.

Here’s an example:

a = "Result: "
b = 2
c = 3
print(a + b + c)

Output: TypeError: can only concatenate str (not “int”) to str

As shown in the snippet, attempting to concatenate a string with integers directly causes a TypeError. This highlights that a + b is not universally equivalent to numeric addition and requires type-specific handling.

Method 2: Operator Overloading with Objects

In Python, classes can define their own behavior for the + operator through the __add__ method. When instances of such classes are added together, the result of a + b does not necessarily equate to the sum of two numbers, but rather the result of a custom defined behavior.

Here’s an example:

class Vector:
    def __init__(self, x):
        self.x = x
    def __add__(self, other):
        return Vector(self.x + other.x)

a = Vector(1)
b = Vector(2)
result = a + b

Output: 3

This code defines a Vector class that overloads the + operator. When two instances of this class are added, the result is a new Vector object with an x attribute equal to the sum of the x attributes of the operands. This demonstrates that in operator overloading, a + b results in whatever the developers define it to be.

Method 3: Implicit Type Conversion (Coercion)

Python sometimes automatically converts one data type to another during operations. This implicit type conversion, known as coercion, can lead to results where a + b does not conform to the expected output of similar type addition if the coercion simplifies the operation to a common data type.

Here’s an example:

a = 10
b = 2.5
result = a + b

Output: 12.5

In this example, the integer a is coerced to a float to perform the addition with float b. The resulting sum, 12.5, is also a float, which demonstrates that coercion can result in a mixed-type addition, leading to a different type as the output.

Method 4: Adding with Complex Numbers

When working with complex numbers in Python, the addition of a and b takes into account both the real and imaginary parts. Consequently, a + b will yield a complex number that combines both parts accordingly, which can be surprising for those expecting a real number result.

Here’s an example:

a = 3 + 4j
b = 1 + 2j
result = a + b

Output: (4+6j)

This snippet demonstrates an addition of two complex numbers. Here, a + b adds both the real parts and the imaginary parts separately, giving a new complex number. This clearly showcases that in the context of complex numbers, addition encompasses more than simple scalar values.

Bonus One-Liner Method 5: Adding Lists and Tuples

Using the + operator with lists or tuples combines them into a larger sequence. Thus, a + b does not sum the individual elements but rather concatenates the sequences, which differs significantly from numerical addition.

Here’s an example:

a = [1, 2]
b = [3, 4]
result = a + b

Output: [1, 2, 3, 4]

As illustrated, adding two lists using the + operator merges them into one larger list. It’s important to understand that this is not element-wise addition but rather a joining of the two sequences, which is a completely different operation in the context of list processing.


  • Method 1: String Concatenation. Pros: Intuitive for combining text. Cons: Can lead to errors if types are not managed properly.
  • Method 2: Operator Overloading with Objects. Pros: Allows for custom behavior that suits the developers’ needs. Cons: Can be confusing if the behavior is not well-documented or understood.
  • Method 3: Implicit Type Conversion. Pros: Simplifies certain operations without explicit type conversion. Cons: May result in unexpected data types.
  • Method 4: Adding Complex Numbers. Pros: Handles complex arithmetic cleanly. Cons: The addition result is not a real number, which may be unexpected.
  • Bonus Method 5: Adding Lists and Tuples. Pros: Quick way to concatenate sequences. Cons: Does not perform element-wise addition, which might be the intended operation.