[BangPypers] Truth Tests (bool)

Sreenivas Reddy T thatiparthysreenivas at gmail.com
Fri Jan 13 17:13:58 CET 2012

I would like to clear things here.I am assuming beginner as  a common
denominator to this post.

len() method is  just a shortcut to __len__ magic method .So when you  call
len() on your own container type ,it does not traverse whole sequence ,it
just calls __len__ method and returns value(assume self.size=0 in
constructor) from this method.This value incremented or decremented in  add
and remove method on your container type.

Moreover,there can never be length method on iterators - (generators are
also iterators,not vice versa).Iterators are meant for traversing a
sequence or container,they do not keep track of length..generators are
meant to produce infinite items.

When you call len(list) or len(deque) or any collection type,they override
this __len__ internally or some other similar mechanism .That is very much
“similar” to ArrayList here in java.


the same can be seen here ..


To make your type iterable ,you just have to implement __iter__ and next()
method,there is  no __len__ method here.So you will always receive " there
is no len() method your object method".Since you passed generator object to
it.you received that error.

This whole discussion is equal to IEnumerable<T> and ICollection<T> in C#.

python gist is here ,https://gist.github.com/1607183

anybody can test it.

Thanks & Regards,
Sreenivas Reddy Thatiparthy,

"Anyone who has never made a mistake has never tried anything new !!! "
--Albert Einstein

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