using zip() and dictionaries

Sneaky Wombat joe.hrbek at
Thu Apr 30 19:45:23 CEST 2009

I'm really confused by what is happening here.  If I use zip(), I
can't update individual dictionary elements like I usually do.  It
updates all of the dictionary elements.  It's hard to explain, so here
is some output from an interactive session:

In [52]: header=['a','b','c','d']
In [53]: columnMap={}
In [54]: for (k,v) in zip(header,[[]]*len(header)):
   ....:     #print "%s,%s"%(k,v)
   ....:     columnMap[k] = v
In [55]: columnMap
Out[55]: {'a': [], 'b': [], 'c': [], 'd': []}
In [56]: columnMap['a'].append('something')
In [57]: columnMap
{'a': ['something'],
 'b': ['something'],
 'c': ['something'],
 'd': ['something']}

Why does ['something'] get attached to all columnMap elements instead
of just element 'a'?

In [58]: columnMap={'a': [], 'b': [], 'c': [], 'd': []}
In [59]: columnMap['a'].append('something')
In [60]: columnMap
Out[60]: {'a': ['something'], 'b': [], 'c': [], 'd': []}

creating the dictionary without using zip, it works as normal.

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