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    <p>MyHDL is using the <code>yield</code> statement to communicate a list of conditions which, when one of them is <code>True</code>, will resume execution of the generator. For example, a generator might yield condition <code>clock.posedge</code> when the clock transitions from low to high (0 to 1) -- when the clock makes this transition, the generator will be resumed.</p> <p>To simulate (roughly) how this works, here is a Python generator which is resumed when one of its conditions (argument is evenly divisible by 3 or 7) are met:</p> <pre><code>def process(): j = 0 while True: yield (lambda x: x % 3 == 0, lambda x: x % 7 == 0) j += 1 print 'process j=', j gen = process() conds = next(gen) for i in range(1, 11): print 'i=', i if any(cond(i) for cond in conds): conds = next(gen) </code></pre> <p>Output:</p> <pre><code>i= 1 i= 2 i= 3 process j= 1 i= 4 i= 5 i= 6 process j= 2 i= 7 process j= 3 i= 8 i= 9 process j= 4 i= 10 </code></pre> <p><strong>Update</strong> - A bit more detail on some of the syntax I used:</p> <p>I tend to use the [<code>next(gen, [default])</code>] function as it is a bit more flexible than calling <code>gen.next()</code>. For example, if you pass the <code>default</code> arg, when the generator is exhausted it will return <code>default</code> rather than <a href="http://docs.python.org/library/exceptions.html#exceptions.StopIteration" rel="nofollow">raising <code>StopIteration</code></a>.</p> <p>The var <code>conds</code> points to a <a href="http://www.python.org/doc//current/library/functions.html?highlight=tuple#tuple" rel="nofollow">tuple</a> containing the conditions. In this case above it points to a <a href="http://www.python.org/doc//current/library/functions.html?highlight=tuple#tuple" rel="nofollow">tuple</a> containing the 2 <a href="http://www.python.org/doc//current/reference/expressions.html#lambda" rel="nofollow">lambda anonymous functions</a> returned by <code>process</code>.</p> <p>The syntax:</p> <pre><code>if any(cond(i) for cond in conds): conds = next(gen) </code></pre> <p>Calls the <a href="http://docs.python.org/library/functions.html#any" rel="nofollow"><code>any</code> built-in function</a>, passing it a <a href="http://www.python.org/dev/peps/pep-0289/" rel="nofollow">generator expression</a> which loops over <code>conds</code> and evaluates <code>if cond(i) is True</code>. It is shorthand for writing:</p> <pre><code>for cond in conds: if cond(i): conds = next(gen) break </code></pre>
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    1. CO@samplebias : Thanks a lot gor you reply...however could you please explain the following in you code: 1)conds=next(gen) means conds=gen.next()..right? 2)could you explain what does "for cond in conds" mean...conds is not a list as far as I can interpret. So how are we iterating over it? Thanks a lot!
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    2. CO@samplebias: Thanks for your reply!..ignore my last post...i went through your example and understood it properly!..however I still dont get the answer...in python we have to explicitly call next(conds) when we want to invoke the function again...in myhdl thats not the case....it automatically triggers itself when the object in the yield statement "fires back"(this is what they have said in their documentation)... how is this implemented?
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    3. CO@Adwait I believe they are using the Python code to generate HDL which behaves differently. Compare some [MyHDL Python source](http://www.myhdl.org/doku.php/cookbook:sinecomp#design) with the corresponding [generated Verilog code](http://www.myhdl.org/doku.php/cookbook:sinecomp#automatic_conversion_to_verilog). Notice how the `yield clock.posedge` creates the `always(..)` directive.
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