[Python-Dev] PEP 3144 review.

[Tres Seaver]

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Scott Dial wrote:

> The purpose is to avoid conflating IPNetwork with an IPAddress that has
> a mask.

I'm not confused by anything in this discussion except the repeated
harping on the (to me imaginary) concept of "address with a mask".
Conceptually:

 - Addresses don't have masks:  they are just 32- (128-) bit integers.

 - Any given address can belong to a number of networks, based on a
   given topology.

 - Masks are also 32- (128-) bit integers, which happen to have the
   property that their leftmost N bits are all zero and the rest are all
   one.

 - One can construct a network from the combination of an address and
   a mask.  Such a network *never* needs to know what the address was
   before masking.

 - Networks *do* know their "zero" address, as well as their "last
   address", and can do "containment" operations on addresses (I'm not
   clear on the need to do such operations on other networks, rather
   than just using their zero addresses).

At the API level, it is convenient to represent the combination
(address, mask) as a single string (e.g., '192.168.1.0/24'):  parsing
that string should return a network, not an address.  Let's keep the
"parsing human readable / writable strings" part separate from the clear
conceptual map.

> If the IPNetwork didn't accept a non-zero host and instead
> required a developer to use a helper to construct a IPNetwork with a
> proper address, then there would be less confusion about what exactly a
> IPNetwork is meant to represent. As it stands, it's purposes is muddled
> by accepting host addresses too.

I can't imagine why it would be useful to try to infer a network from an
address:  "In the face of ambiguity, refuse the temptation to guess."

> I am rather indifferent whether there needs to be a IPAddressWithMask
> type. If that is needed, then it is rather easy to create a type that
> does that. And, if it is a common pattern, then it could be added to the
> module later in life.

Agreed.


Tres.
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Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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