DataFrame with 'list of dicts' behaviour proposal

[gregglind]

Sketch of proposed behaviour... make 'list of dicts' create a (potentially) 'ragged' array, with autoguessed column names, and sensible default values, when the keys don't exist in all dicts.

Current behaviour:

In [215]: pandas.DataFrame([dict(a=1),dict(a=2)],columns=['a'])
Out[215]:
a
0 {'a': 1}
1 {'a': 2}

(I happen to find this very surprising/useless behaviour!)

(one) Proposed behaviour...

print DataFrame2([dict(a=1,c=1,d=True),dict(b=2,c='abc')])
a c d
0 1 1 True
1 NaN abc NaN

I have a straw implementation at: https://gist.github.com/1511578

(there is a lot to comment on!... should it use the set of keys? Do we need more args? Documentation? Is this just a recipe?)

[wesm]

Let me guess...you've got lists of JSON objects? =P

this works for example:

In [4]: DataFrame.from_dict(dict(zip(range(2), [dict(a=1,c=1,d=True),dict(b=2,c='abc')])), orient='index')
Out[4]: 
   a    b    c    d  
0  1    NaN  1    1  
1  NaN  2    abc  NaN

but I agree with you that the constructor should be able to figure out a list of dicts without having to type so much. I'll look at your impl and cook up something similar / fast as possible.

[gregglind]

My data does mostly come in from JSON, and I have to transform it. I
wrote a mutant jsonpath / jquery sort of of way to 'flatten' out
json/mongo structures into 'table-ish' (row-column) things, which is
already gross enough! My current temptation is to use R (because, for
the life of me, I don't grok numpy indexing / slicing), but pandas
DataFrame feels right :)

Eventually, I want to make my exploratory stuff as simple as possible,
as described in previous rants!

Note: If the default 'use all columns that appear in any' is desired
(which feels 'more right' to me):

        from itertools import chain
        columns = sorted(
            set(chain(*(x.keys() for x in data)))
        )

It's worth thinking about if this is something you want to actually
include. It think this is fixable only patching
pandas/core/frame.py:DataFrame, docs, and tests. That
whole set of code could stand an interface / behavior / documentation
review. Lots of isinstance and other hidden assumptions (like
tending to privilege whatever goes on in row[0]).

Thanks for reviewing the idea! (and sorry that the iget is so
gross! It should have to hide in utils as punishment)

GL

On Thu, Dec 22, 2011 at 2:07 PM, Wes McKinney
[email protected]
wrote:

Let me guess...you've got lists of JSON objects? =P

this works for example:

In [4]: DataFrame.from_dict(dict(zip(range(2), [dict(a=1,c=1,d=True),dict(b=2,c='abc')])), orient='index')
Out[4]:
  a    b    c    d
0  1    NaN  1    1
1  NaN  2    abc  NaN

but I agree with you that the constructor should be able to figure out a list of dicts without having to type so much. I'll look at your impl and cook up something similar / fast as possible.

---

Reply to this email directly or view it on GitHub:
https://github.com/wesm/pandas/issues/526#issuecomment-3254153

[wesm]

I think 'use all columns that appear in any' is the right default behavior unless a set of columns is explicitly passed (in which case obviously just use those). This would probably also be a good time to review all the dict-creation routines and set up some vbench action for them too (http://pandas.sourceforge.net/vbench.html). I'm kind of performance obsessed (!) if that hasn't come through yet, so I suspect I can come up with a pretty performant way of processing the data into the right form.

As far as giving privilege to the first element of a list...well, if a user passes a list of differently-typed objects, that is most likely going to blow up. In practice that is pretty rare so I'm willing to live with it.

[gregglind]

Let me know if you want design or code review on any of it! I will be
posting my jsonpath-ish stuff soon, which is allied to pandas.

(eventually, I want to write bridge code to use DataFrames in orange as well)

GL

On Thu, Dec 22, 2011 at 2:32 PM, Wes McKinney
[email protected]
wrote:

I think  'use all columns that appear in any' is the right default behavior unless a set of columns is explicitly passed (in which case obviously just use those). This would probably also be a good time to review all the dict-creation routines and set up some vbench action for them too (http://pandas.sourceforge.net/vbench.html). I'm kind of performance obsessed (!) if that hasn't come through yet, so I suspect I can come up with a pretty performant way of processing the data into the right form.

As far as giving privilege to the first element of a list...well, if a user passes a list of differently-typed objects, that is most likely going to blow up. In practice that is pretty rare so I'm willing to live with it.

---

Reply to this email directly or view it on GitHub:
https://github.com/wesm/pandas/issues/526#issuecomment-3254449

[wesm]

Cool. I think that would be very valuable (on both fronts). I'd be happy to have json-related tools in pandas, I'm eventually going to need to write up DataFrame with JS data visualization in the browser

[wesm]

I implemented this in the above commit. I guess you piqued my interest :) btw the implementation (utilizing Cython routines) above is roughly 6x faster than the one in the gist above. The Cython routine I have that implements

from itertools import chain
columns = sorted(
    set(chain(*(x.keys() for x in data)))
)

beats it by about 35%. Though I do love the simple elegance of itertools and generators