What criteria can be used to determine code quality and distinguish between bad and good code?

Code quality can be assessed based on several criteria:

• Readability: the code should be understandable by other developers, with meaningful variable and function names.
• Maintainability: it should be easy to make changes and fix bugs without risking breaking other parts.
• Simplicity: avoid unnecessary complexity, use clear constructs.
• Reusability: the code should be modular and reusable.
• Testability: the code should be easy to cover with tests.
• Performance: the code should work efficiently, but not at the expense of readability.

Poor code often:

• contains duplication,
• is poorly structured,
• uses unclear names,
• is undocumented,
• is difficult to understand and modify.

An example of good practice in Python:

def calculate_area(radius):
    """Calculates the area of a circle given the radius."""
    import math
    return math.pi * radius ** 2

Here, the function is short, with a clear name and documentation.