[Tutor] Behavior of dictionary in mapping keys that evaluate equal

[Steven D'Aprano]

On Wed, May 11, 2016 at 02:00:47PM +0100, khalil zakaria Zemmoura wrote:
> Hi,
> 
> Suppose we have a dict
> Dic = { True: 'yes', 1: 'No'}
> 
> According to the Python 3 documentation, the keys must have a unique value
> so True is converted to integer because of the type coercion (boolean are
> subtype of integer) so boolean are winded to integer to be compared.

No, not quite. Keys aren't converted to anything. The above is 
equivalent to:

Dic = {}
Dic[True] = 'yes'
Dic[1] = 'no'

That part is straight-forward. But remember that True == 1:

py> True == 1
True

and they have the same hash:

py> hash(True), hash(1)
(1, 1)

so as far as Python is concerned, they are the same key. The same 
applies to the float 1.0:

py> d = {True: 'yes'}
py> d[1]
'yes'
py> d[1.0]
'yes'


One last fact to remember: if the key is already found in the dict, it 
isn't replaced. So:

py> d = {1: 'no'}
py> d[True] = 'yes'
py> d
{1: 'yes'}


Does this now explain why

{True: 'yes', 1: 'no'}

returns {True: 'no'}?


-- 
Steve