save dictionary to a file without brackets.

[88888 Dihedral]

Steven D'Aprano於 2012年8月11日星期六UTC+8下午7時26分37秒寫道：
> On Fri, 10 Aug 2012 08:53:43 +1000, Chris Angelico wrote:
> 
> 
> 
> > On Fri, Aug 10, 2012 at 8:26 AM, Dave Angel <d at davea.name> wrote:
> 
> >> On 08/09/2012 06:03 PM, Andrew Cooper wrote:
> 
> >>> O(n) for all other entries in the dict which suffer a hash collision
> 
> >>> with the searched entry.
> 
> >>>
> 
> >>> True, a sensible choice of hash function will reduce n to 1 in common
> 
> >>> cases, but it becomes an important consideration for larger datasets.
> 
> >>
> 
> >> I'm glad you're wrong for CPython's dictionaries.  The only time the
> 
> >> lookup would degenerate to O[n] would be if the hash table had only one
> 
> >> slot.  CPython sensibly increases the hash table size when it becomes
> 
> >> too small for efficiency.
> 
> >>
> 
> >> Where have you seen dictionaries so poorly implemented?
> 
> > 
> 
> > In vanilla CPython up to version (I think) 3.3, where it's possible to
> 
> > DoS the hash generator. Hash collisions are always possible, just
> 
> > ridiculously unlikely unless deliberately exploited.
> 
> 
> 
> Not so unlikely actually.
> 
> 
> 
> py> hash(3)
> 
> 3
> 
> py> hash(2**64 + 2)
> 
> 3
> 
> 
> 
> py> hash(-1)
> 
> -2
> 
> py> hash(-2)
> 
> -2
> 
> 
> 
> 
> 
> By its very nature, a hash function cannot fail to have collisions. The 
> 
> problem is that in general you have an essentially unlimited number of 
> 
> objects being mapped to a large but still finite number of hash values.
> 
> 
> 
> 
> 
> 
> 
> -- 
> 
> Steven



Steven D'Aprano於 2012年8月11日星期六UTC+8下午7時26分37秒寫道：
> On Fri, 10 Aug 2012 08:53:43 +1000, Chris Angelico wrote:
> 
> 
> 
> > On Fri, Aug 10, 2012 at 8:26 AM, Dave Angel <d at davea.name> wrote:
> 
> >> On 08/09/2012 06:03 PM, Andrew Cooper wrote:
> 
> >>> O(n) for all other entries in the dict which suffer a hash collision
> 
> >>> with the searched entry.
> 
> >>>
> 
> >>> True, a sensible choice of hash function will reduce n to 1 in common
> 
> >>> cases, but it becomes an important consideration for larger datasets.
> 
> >>
> 
> >> I'm glad you're wrong for CPython's dictionaries.  The only time the
> 
> >> lookup would degenerate to O[n] would be if the hash table had only one
> 
> >> slot.  CPython sensibly increases the hash table size when it becomes
> 
> >> too small for efficiency.
> 
> >>
> 
> >> Where have you seen dictionaries so poorly implemented?
> 
> > 
> 
> > In vanilla CPython up to version (I think) 3.3, where it's possible to
> 
> > DoS the hash generator. Hash collisions are always possible, just
> 
> > ridiculously unlikely unless deliberately exploited.
> 
> 
> 
> Not so unlikely actually.
> 
> 
> 
> py> hash(3)
> 
> 3
> 
> py> hash(2**64 + 2)
> 
> 3
> 
> 
> 
> py> hash(-1)
> 
> -2
> 
> py> hash(-2)
> 
> -2
> 
> 
> 
> 
> 
> By its very nature, a hash function cannot fail to have collisions. The 
> 
> problem is that in general you have an essentially unlimited number of 
> 
> objects being mapped to a large but still finite number of hash values.
> 
> 
> 
> 
> 
> 
> 
> -- 
> 
> Steven

Lets check the basic operations of a hash table or so-called a dictionary first.

If the dictionary is implemented toward faster in searching items, 
then it is slightly different in the insertion and the deletion operations of
(key, value) pairs.