Pandas MultiIndex lookup with Numpy arrays


I'm working with a pandas DataFrame that represents a graph. The dataframe is indexed by a MultiIndex that indicates the node endpoints.


import pandas as pd
import numpy as np
import itertools as it
edges = list(it.combinations([1, 2, 3, 4], 2))

# Define a dataframe to represent a graph
index = pd.MultiIndex.from_tuples(edges, names=['u', 'v'])
df = pd.DataFrame.from_dict({
    'edge_id': list(range(len(edges))),
    'edge_weight': np.random.RandomState(0).rand(len(edges)),
df.index = index
## -- End pasted text --
     edge_id  edge_weight
u v                      
1 2        0       0.5488
  3        1       0.7152
  4        2       0.6028
2 3        3       0.5449
  4        4       0.4237
3 4        5       0.6459

I want to be able to index into the graph using an edge subset, which is why I've chosen to use a MultiIndex. I'm able to do this just fine as long as the input to df.loc is a list of tuples.

# Select subset of graph using list-of-tuple indexing
edge_subset1 = [edges[x] for x in [0, 3, 2]]
## -- End pasted text --
     edge_id  edge_weight
u v                      
1 2        0       0.5488
2 3        3       0.5449
1 4        2       0.6028

However, when my list of edges is a numpy array (as it often is), or a list of lists, then I seem to be unable to use the df.loc property.

# Why can't I do this if `edge_subset2` is a numpy array?
edge_subset2 = np.array(edge_subset1)
## -- End pasted text --
TypeError: unhashable type: 'numpy.ndarray'

It would be ok if I could just all arr.tolist(), but this results in a seemingly different error.

# Why can't I do this if `edge_subset2` is a numpy array?
# or if `edge_subset3` is a list-of-lists?
edge_subset3 = edge_subset2.tolist()
## -- End pasted text --
TypeError: '[1, 2]' is an invalid key

It's a real pain to have to use list(map(tuple, arr.tolist())) every time I want to select a subset. It would be nice if there was another way to do this.

The main questsions are:

  • Why can't I use a numpy array with .loc? Is it because under the hood a dictionary is being used to map the multi-index labels to positional indices?

  • Why does a list-of-lists give a different error? Maybe its really the same problem its just caught a different way?

  • Is there another (ideally less-verbose) way to lookup a subset of a dataframe with a numpy array of multi-index labels that I'm unaware of?

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| numpy   | pandas   | python   2017-01-05 20:01 0 Answers

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