Updating python dictionary

[andyhume@gmail.com]

Hello...

I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:

MAPPING_DICT = {
'a': 'A',
'b': 'B',
}

my_dict = {
'a': '1',
'b': '2'
}

I want the finished my_dict to look like:

my_dict = {
'A': '1',
'B': '2'
}

Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.

Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

Cheers,
Andy.

[Marc 'BlackJack' Rintsch]

Re: Updating python dictionary

On Sun, 07 Sep 2008 14:51:32 -0700, andyhume@gmail. com wrote:

MAPPING_DICT = {
'a': 'A',
'b': 'B',
}
>
my_dict = {
'a': '1',
'b': '2'
}
>
I want the finished my_dict to look like:
>
my_dict = {
'A': '1',
'B': '2'
}
>
Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.
>
Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

You only have to loop through `my_dict`:

In [187]: %cpaste
Pasting code; enter '--' alone on the line to stop.
:MAPPING_DICT = {
: 'a': 'A',
: 'b': 'B',
:}
:
:my_dict = {
: 'a': '1',
: 'b': '2'
:}
:--

In [188]: dict((MAPPING_D ICT[k], v) for k, v in my_dict.iterite ms())
Out[188]: {'A': '1', 'B': '2'}

Ciao,
Marc 'BlackJack' Rintsch

[MK Bernard]

Re: Updating python dictionary

On Sep 7, 2:51 pm, "andyh...@gmail .com" <andyh...@gmail .comwrote:

Hello...
>
I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:
>
MAPPING_DICT = {
    'a': 'A',
    'b': 'B',
>
}
>
my_dict = {
    'a': '1',
    'b': '2'
>
}
>
I want the finished my_dict to look like:
>
my_dict = {
    'A': '1',
    'B': '2'
>
}
>
Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.
>
Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?
>
Cheers,
Andy.

You could accomplish this with something similar to the following:
new_dict = {}
for x in MAPPING_DICT.ke ys():
if x in my_dict.keys():
new_dict[MAPPING_DICT[x]] = my_dict[x]

Although there is probably a better solution to the problem your
having. Perhaps more details could help us lead in you the right
direction?

Cheers, MK

[Fredrik Lundh]

Re: Updating python dictionary

andyhume@gmail. com wrote:

Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

since none of your dictionaries contain the keys you want in the final
dictionary, creating a brand new dict sounds pretty clever to me.

</F>

[James Mills]

Re: Updating python dictionary

Hi,

There is "never" a "clever" way
of doing anything, but:

$ cat test.py
MAPPING_DICT = {'a': 'A','b': 'B',}
my_dict = {'a': '1','b': '2'}
my_dict = dict((MAPPING_D ICT[k], my_dict[k]) for k in my_dict)
print my_dict
$ python test.py
{'A': '1', 'B': '2'}
$

That should do the trick.

cheers
James

On Mon, Sep 8, 2008 at 7:51 AM, andyhume@gmail. com <andyhume@gmail .comwrote:

Hello...
>
I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:
>
MAPPING_DICT = {
'a': 'A',
'b': 'B',
}
>
my_dict = {
'a': '1',
'b': '2'
}
>
I want the finished my_dict to look like:
>
my_dict = {
'A': '1',
'B': '2'
}
>
Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.
>
Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?
>
Cheers,
Andy.
--

http://mail.python.org/mailman/listinfo/python-list

>

--
--
-- "Problems are solved by method"

[Gabriel Genellina]

Re: Updating python dictionary

En Sun, 07 Sep 2008 18:51:32 -0300, andyhume@gmail. com <andyhume@gmail .comescribió:

I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:
>
MAPPING_DICT = {
'a': 'A',
'b': 'B',
}
>
my_dict = {
'a': '1',
'b': '2'
}
>
I want the finished my_dict to look like:
>
my_dict = {
'A': '1',
'B': '2'
}
>
Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.
>
Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

Exactly. You can do that in one pass:

my_new_dict = dict((MAPPING_D ICT[k],v) for (k,v) in my_dict.iterite ms())

That's enough if MAPPING_DICT always contains all the keys. If you want to keep old keys that aren't in MAPPING_DICT unchanged, use MAPPING_DICT.ge t(k,k) instead. Other corner cases include many-to-one mappings, and incomplete mappings where a replacement key is also an unmapped old key.

--
Gabriel Genellina

[John Machin]

Re: Updating python dictionary

On Sep 8, 7:51 am, "andyh...@gmail .com" <andyh...@gmail .comwrote:

Hello...
>
I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:
>
MAPPING_DICT = {
    'a': 'A',
    'b': 'B',
>
}
>
my_dict = {
    'a': '1',
    'b': '2'
>
}
>
I want the finished my_dict to look like:
>
my_dict = {
    'A': '1',
    'B': '2'
>
}
>
Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.
>
Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?
>

Is this homework?

There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.

If it doesn't include all possible input keys, answers will crash with
a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

[James Mills]

Re: Updating python dictionary

On Mon, Sep 8, 2008 at 8:37 AM, John Machin <sjmachin@lexic on.netwrote:

Is this homework?

I hope it's not - or I'll be quite annoyed :)

There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.
>
If it doesn't include all possible input keys, answers will crash with
a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

You are quite right! But then he/she didn't
ask for this right ? :)

cheers
James

--
--
-- "Problems are solved by method"

[John Machin]

Re: Updating python dictionary

On Sep 8, 8:42 am, "James Mills" <prolo...@short circuit.net.auw rote:

On Mon, Sep 8, 2008 at 8:37 AM, John Machin <sjmac...@lexic on.netwrote:

There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.

>

If it doesn't include all possible input keys, answers will crash with
a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

>
You are quite right! But then he/she didn't
ask for this right ? :)
>

What do you mean by "this right"? Perhaps the Divine Right of OPs,
managers, examiners, business analysts, etc never to give a complete
spec up front and never to contemplate the consequences of Murphy's
Law?

[Steven D'Aprano]

Re: Updating python dictionary

On Sun, 07 Sep 2008 15:59:52 -0700, John Machin wrote:

On Sep 8, 8:42 am, "James Mills" <prolo...@short circuit.net.auw rote:

>On Mon, Sep 8, 2008 at 8:37 AM, John Machin <sjmac...@lexic on.net>
>wrote:

>

There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.

>>

If it doesn't include all possible input keys, answers will crash
with a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

>>
>You are quite right! But then he/she didn't ask for this right ? :)
>>
>>

What do you mean by "this right"? Perhaps the Divine Right of OPs,
managers, examiners, business analysts, etc never to give a complete
spec up front and never to contemplate the consequences of Murphy's Law?

I *think* that what James Mills meant was:

"But then he/she didn't ask for this comma right?"



--
Steven

[James Mills]

Re: Updating python dictionary

On Mon, Sep 8, 2008 at 8:59 AM, John Machin <sjmachin@lexic on.netwrote:

What do you mean by "this right"? Perhaps the Divine Right of OPs,
managers, examiners, business analysts, etc never to give a complete
spec up front and never to contemplate the consequences of Murphy's
Law?

Now you're being silly.

--
--
-- "Problems are solved by method"

[MK Bernard]

Re: Updating python dictionary

On Sep 7, 3:37 pm, John Machin <sjmac...@lexic on.netwrote:

On Sep 8, 7:51 am, "andyh...@gmail .com" <andyh...@gmail .comwrote:
>
>
>

Hello...

>

I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:

>

MAPPING_DICT = {
    'a': 'A',
    'b': 'B',

>

}

>

my_dict = {
    'a': '1',
    'b': '2'

>

}

>

I want the finished my_dict to look like:

>

my_dict = {
    'A': '1',
    'B': '2'

>

}

>

Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.

>

Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

>
Is this homework?
>
There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.
>
If it doesn't include all possible input keys, answers will crash with
a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

Thats exactly why I did an explicit check in my post, so as to make
sure that such a collision could not occur. It would seem that
something along what I posted would be safer, if less elegant, than
the others.
Cheers, MK

[John Machin]

Re: Updating python dictionary

On Sep 8, 10:47 am, MK Bernard <mkbernard....@ gmail.comwrote:

On Sep 7, 3:37 pm, John Machin <sjmac...@lexic on.netwrote:
>
>
>
>
>

On Sep 8, 7:51 am, "andyh...@gmail .com" <andyh...@gmail .comwrote:

>

Hello...

>

I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:

>

MAPPING_DICT = {
    'a': 'A',
    'b': 'B',

>

}

>

my_dict = {
    'a': '1',
    'b': '2'

>

}

>

I want the finished my_dict to look like:

>

my_dict = {
    'A': '1',
    'B': '2'

>

}

>

Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.

>

Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

>

Is this homework?

>

There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.

>

If it doesn't include all possible input keys, answers will crash with
a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

>
Thats exactly why I did an explicit check in my post, so as to make
sure that such a collision could not occur. It would seem that
something along what I posted would be safer, if less elegant, than
the others.

I noted two problems:
(1) not covering all input keys: your code explicitly sweeps this
problem under the carpet, and does it laboriously ...
if x in my_dict.keys():
instead of
if x in my_dict:
(2) not a 1:1 mapping -- for example, 'a' and 'b' both map to 'A' (the
only "collision" that I can see), but you don't address that.

Here's some code which attempts to cover the bases:

new_dict = {}
for key in my_dict:
if key not in MAPPING_DICT:
raise NoMapError('bla h %r' % key)
else:
new_key = MAPPING_DICT[key]
if new_key in new_dict:
raise ManyToOneError( 'gurgle %r waffle %r' % (key, new_key))
else:
new_dict[new_key] = my_dict[key]

Having considered what is actually required under the checked-for
conditions, one or both raise statements may be replaced by other
code, and then simplified e.g. the first 4 lines in the loop may end
up being replaced by
new_key = MAPPING_DICT.ge t(key, key)
as already suggested by one respondent.

Note that useful use-cases for any variety of this key-change exercise
may have many more items in MAPPING_DICT than there are in my_dict (a
big language dictionary and a few fragments of text to be
"translated "), or vice versa (a small synonym dictionary (color ->
colour, center -centre) and a big input to be "standardis ed") so
it's a good idea to inquire which is likely to be the smaller and
iterate over that if the requirements permit it.

[John Machin]

Re: Updating python dictionary

On Sep 8, 9:14 am, "James Mills" <prolo...@short circuit.net.auw rote:

On Mon, Sep 8, 2008 at 8:59 AM, John Machin <sjmac...@lexic on.netwrote:

What do you mean by "this right"? Perhaps the Divine Right of OPs,
managers, examiners, business analysts, etc never to give a complete
spec up front and never to contemplate the consequences of Murphy's
Law?

>
Now you're being silly.
>

Only in so far as I followed the habit of OPs and suchlike by using
"never" instead of "rarely".

[andyhume@gmail.com]

Re: Updating python dictionary

On Sep 8, 10:20 am, John Machin <sjmac...@lexic on.netwrote:

On Sep 8, 10:47 am, MK Bernard <mkbernard....@ gmail.comwrote:
>
>
>

On Sep 7, 3:37 pm, John Machin <sjmac...@lexic on.netwrote:

>

On Sep 8, 7:51 am, "andyh...@gmail .com" <andyh...@gmail .comwrote:

>

Hello...

>

I have a dict of key/values and I want to change the keys in it, based
on another mapping dictionary. An example follows:

>

MAPPING_DICT = {
    'a': 'A',
    'b': 'B',

>

}

>

my_dict = {
    'a': '1',
    'b': '2'

>

}

>

I want the finished my_dict to look like:

>

my_dict = {
    'A': '1',
    'B': '2'

>

}

>

Whereby the keys in the original my_dict have been swapped out for the
keys mapped in MAPPING_DICT.

>

Is there a clever way to do this, or should I loop through both,
essentially creating a brand new dict?

>

Is this homework?

>

There seems to be an implicit assumption in the answers so far that
your mapping is a 1:1 mapping of all possible input keys.

>

If it doesn't include all possible input keys, answers will crash with
a KeyError. If there are any many:1 elements in the mapping (for
example, {'a': 'A', 'b': 'A'}), lossage happens. You may wish to code
in some checks for this.

>

Thats exactly why I did an explicit check in my post, so as to make
sure that such a collision could not occur. It would seem that
something along what I posted would be safer, if less elegant, than
the others.

>
I noted two problems:
(1) not covering all input keys: your code explicitly sweeps this
problem under the carpet, and does it laboriously ...
    if x in my_dict.keys():
instead of
    if x in my_dict:
(2) not a 1:1 mapping -- for example, 'a' and 'b' both map to 'A' (the
only "collision" that I can see), but you don't address that.
>
Here's some code which attempts to cover the bases:
>
new_dict = {}
for key in my_dict:
    if key not in MAPPING_DICT:
        raise NoMapError('bla h %r' % key)
    else:
        new_key = MAPPING_DICT[key]
    if new_key in new_dict:
        raise ManyToOneError( 'gurgle %r waffle %r' % (key, new_key))
    else:
        new_dict[new_key] = my_dict[key]
>
Having considered what is actually required under the checked-for
conditions, one or both raise statements may be replaced by other
code, and then simplified e.g. the first 4 lines in the loop may end
up being replaced by
    new_key = MAPPING_DICT.ge t(key, key)
as already suggested by one respondent.
>
Note that useful use-cases for any variety of this key-change exercise
may have many more items in MAPPING_DICT than there are in my_dict (a
big language dictionary and a few fragments of text to be
"translated "), or vice versa (a small synonym dictionary (color ->
colour, center -centre) and a big input to be "standardis ed") so
it's a good idea to inquire which is likely to be the smaller and
iterate over that if the requirements permit it.

In the interests of academia (although this isn't homework :)) I'll
answer some of those questions:

- Yes, the mapping dict doesn't necessarily have the same number of
entries as the my_dict. It may have less than it or more than it.
- There will never be dupes of key or value in the mapping. Though
there maybe in my_dict.

And, for extra credit, it may be useful to be able to 'translate' the
my_dict back again using the same mapping_dict. ie, using the values
in mapping dict as the new keys in my_dict, and the keys as the new
values.

Andy.