FunctionStatementNestingDepth.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
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 <code>if</code>
statements.</p>

</overview>
<recommendation>

<p>
Extract the control flow into a separate generator and use that to control iteration.</p>

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

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

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

<p>
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.
</p>

</recommendation>
<example>

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

<sample src="FunctionStatementNestingDepth.py" />

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

<sample src="FunctionStatementNestingDepthGood.py" />

</example>
<references>


<li>
M. Fowler, <em>Refactoring</em>, pp. 89-95. Addison-Wesley, 1999.
</li>
<li>
S. McConnell, <em>Code Complete</em>, 2nd Edition, &sect;19.4. Microsoft Press, 2004.
</li>


</references>
</qhelp>