Why Does Python’s bar(3+2) Return 5? A Deep Dive into Function Arguments

1. Introduction

A question about a Python function calculation was raised in a community group, prompting an explanation of how arguments are passed and defaults are applied.

2. Implementation

The solution breaks the problem into two functions, bar and foo, and demonstrates their interaction.

def bar(z, x=0):
    return z + x

def foo(x):
    return bar(3+x)

print(foo(2))

Because bar(3+2) receives only one positional argument, the second parameter x uses its default value 0, resulting in 3+2+0 = 5.

The expression 3+2 is evaluated at the point where bar is called, and the resulting value is passed as z.

To visualize the execution, the online tool Python Tutor can be used, providing a step‑by‑step view similar to setting breakpoints in an IDE.

3. Summary

The article walks through a Python function argument problem, explains why the default parameter is used, and provides code and visualization resources to help readers solve similar issues.