votes up 13

Names must be a list-like

Package:
pandas
github stars 30911
Exception Class:
ValueError

Raise code

""" 
            for all levels).  Otherwise level must be None

        Raises
        ------
        TypeError if each name is not hashable.
        """
        if not is_list_like(values):
            raise ValueError("Names must be a list-like")
        if len(values) != 1:
            raise ValueError(f"Length of new names must be 1, got {len(values)}")

        # GH 20527
        # All items in 'name' need to be hashable:
        validate_all_hashable(*values, error_name=f"{type(self).__name__}.name")
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Links to the raise (1)

https://github.com/pandas-dev/pandas/blob/b3e335254f46a526ee3ce9bb757eac4011d9d1fe/pandas/core/indexes/base.py#L1517

Ways to fix

votes up 4 votes down

pandas.MultiIndex.from_arrays is used to convert arrays to MultiIndex. It takes the array and the names as a parameter. The names parameter should be given an array like object of strings.


Reproducing the error:

pipenv install pandas


import pandas as pd
import numpy as np
arrays = [[1, 1, 2, 2], ['red', 'blue', 'red', 'blue']]
df = pd.MultiIndex.from_arrays(arrays, names='number')
print(df)

The error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-10-9aae93f15a38> in <module>()
      2 import numpy as np
      3 arrays = [[1, 1, 2, 2], ['red', 'blue', 'red', 'blue']]
----> 4 df = pd.MultiIndex.from_arrays(arrays, names='number')
      5 print(df)

/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/multi.py in from_arrays(cls, arrays, sortorder, names)
    446             sortorder=sortorder,
    447             names=names,
--> 448             verify_integrity=False,
    449         )
    450 

/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/multi.py in __new__(cls, levels, codes, sortorder, names, dtype, copy, name, verify_integrity, _set_identity)
    282         if names is not None:
    283             # handles name validation
--> 284             result._set_names(names)
    285 
    286         if sortorder is not None:

/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/multi.py in _set_names(self, names, level, validate)
   1340         # Don't allow a single string for names in a MultiIndex
   1341         if names is not None and not is_list_like(names):
-> 1342             raise ValueError("Names should be list-like for a MultiIndex")
   1343         names = list(names)
   1344 

ValueError: Names should be list-like for a MultiIndex

Fixed version of the code:

import pandas as pd
import numpy as np
arrays = [[1, 1, 2, 2], ['red', 'blue', 'red', 'blue']]
df = pd.MultiIndex.from_arrays(arrays, names=('number', 'color'))
print(df)

Output:

MultiIndex([(1,  'red'),
            (1, 'blue'),
            (2,  'red'),
            (2, 'blue')],
           names=['number', 'color'])

Jul 17, 2021 kellemnegasi answer
kellemnegasi 31.6k

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