Remove keys from object not in a list in python?

Question

I have these keys:

keep = ["a","c"]

My dict:

testdict = {'
'a':'vala',
'b':'valb',
'c':'valc',
'd':'vald'
}

Desired output:

testdict = {
'a':'vala',
'c':'valc'
}

I want to remove all keys that do not match the key in the list. What is the fastest way to do this?

I tried:

for key, value in testdict.iteritems():
      if key not in keep:
         del testdict[key]

But the above gives me errors since the size is changing.

Answer 1

Instead of removing, simply construct a new dict:

newdict = {k: testdict[k] for k in keep}

If keep can contain keys that are not present in testdict, add the appropriate condition:

newdict = {k: testdict[k] for k in keep if k in testdict}

If you absolutely must modify the dict object, bear in mind that you can not modify a dictionary while you're traversing over it. So traverse first and delete afterwards:

to_delete = set(testdict.keys()).difference(keep)
for d in to_delete:
    del testdict[d]

Answer 2

print({k:testDict[k] for k in keep})

or in py<2.7

print dict((k,testDict[k]) for k in keep)

these assume every key in keep is guaranteed to exist in testDict

Answer 3

The problem is you're mutating the keys while using the iterator. If you had a proper list of the keys you could avoid the issue..

for key in list(testdict.keys()):
      if key not in keep:
         del testdict[key]