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Date: Tue, 30 Jul 2013 13:44:47 -0700
From: Gary Herron <gary.herron@islandtraining.com>
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Subject: Re: binary key in dictionary
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On 07/30/2013 01:29 PM, cerr wrote:
> Hi,
>
> In my application I have followingf lines:
>                  print curr_mac
>                  print hexlify(buf)
>                  binmac = unhexlify(curr_mac)
>                  tmpgndict[binmac] += buf
> curr_mac being a 3Byte MAVC address in ASCII and I want to populate a dictionary where the value(buf) is indexed by binary mac.
>
> I get this in my code:
>
> Traceback (most recent call last):
>    File "gateway.py", line 2485, in <module>
>      main()
>    File "gateway.py", line 2459, in main
>      cloud_check()
>    File "gateway.py", line 770, in cloud_check
>      gnstr_dict[src] = gn_from_cloud(curr_mac)
>    File "gateway.py", line 2103, in gn_from_cloud
>      tmpgndict[binmac] += "HELLO"
> KeyError: '\x04\xeeu'
>
> but then again, the following works fine in the python interpreter:
>>>> mac = '04ee75'
>>>> dat = '2a0001016d03c400040001000a'
>>>> mydict = {}
>>>> mydict[unhexlify(mac)]=dat
>>>> print mydict
> {'\x04\xeeu': '2a0001016d03c400040001000a'}
>
> I really seem to do something wrong and can't see what it is. Can anyone help me further here?
>
> Thank you very much!
> Ron


You are confusing the problem with excess code.  Examine the following 
simpler example which illustrates the problem:
 >>> d = {}
 >>> d[1] = 99
 >>> d[2] += 98
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
KeyError: 2
 >>>

The  line
   d[1] = 99
creates a key-value pair in the dictionary, but the line
   d[2] += 98
tries to add 98 to an already existing value at d[2],   But there is no 
value at d[2] until you set it:
   d[2] = 0 # for instance

You may want to look at defaultdict from the collections module.

Gary Herron