Why is a function/method call in python expensive?

Question

In this post, Guido van Rossum says that a function call may be expensive, but I do not understand why nor how much expensive can be.

How much delay adds to your cod

Answer 1

Any statement of the form "X is expensive" doesn't take into account that performance is always relative to whatever else is going on, and relative to however else the task can be done.

There are many questions on SO that express concern about things that might be, but typically are not, performance problems.

As to whether function calls are expensive, there's a general two-part answer.

1. For functions that do very little and do not call further sub-functions, and that in a particular application are responsible for more than 10% of total wall-clock time, it is worthwhile trying to in-line them or otherwise reduce the cost of invocation.

2. In applications containing complex data structures and/or tall abstraction hierarchies, function calls are expensive not because of the time they take, but because they tempt you to make more of them than strictly necessary. When this occurs over multiple levels of abstraction, the inefficiencies multiply together, producing a compounded slowdown that is not easily localized.

The way to produce efficient code is a posteriori, not a priori. First write the code so it is clean and maintainable, including function calls as you like. Then while it is running with a realistic workload, let it tell you what can be done to speed it up. Here's an example.