A method that contains a high level of nesting can be very difficult to understand. As noted in [McConnell], the human brain cannot easily handle more than three levels of nested if statements.

Extract the control flow into a separate generator and use that to control iteration.

Use early exits to move nested statements out of conditions. For example: def func(x): if x: long_complex_block() can be replaced by def func(x): if x: return long_complex_block()

Extract nested statements into new functions, for example by using the 'Extract Method' refactoring from [Fowler].

For more ways to reduce the level of nesting in a method, see [McConnell].

Furthermore, a method that has a high level of nesting often indicates that its design can be improved in other ways, as well as dealing with the nesting problem itself.

In the following example, the code has four levels of nesting and is unnecessarily difficult to read.

In the following modified example, three different approaches to reducing the nesting depth are shown. The first, print_character_codes_early_exit, uses early exits, either return or continue. The second, print_character_codes_use_gen, extracts the control flow into a generator. The third, print_character_codes_extracted, uses a separate function for the inner loop.

M. Fowler, Refactoring, pp. 89-95. Addison-Wesley, 1999.

S. McConnell, Code Complete, 2nd Edition, §19.4. Microsoft Press, 2004.