DataFrame.merge with Series should give nice error message

[dan-tee]

Right now trying this results in "IndexError: list index out of range". It should say can't merge DataFrame with a Series...

I know this for quite a while now, but still get trapped on it every once in a while. This would be very helpful for beginners.

Other people also get confused: http://stackoverflow.com/questions/27281734/pandas-merge-on-index-not-working

[benjello]

👍

[jreback]

can you post a copy-pastable example that shows the error message.

[halflings]

import pandas as pd
df = pd.DataFrame(dict(a=range(20)))
pd.merge(df, df.a, left_index=True, right_index=True)

This gives:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-9-fae8bf77732c> in <module>()
      1 import pandas as pd
      2 df = pd.DataFrame(dict(a=range(20)))
----> 3 pd.merge(df, df.a, left_index=True, right_index=True)

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator)
     33                          right_index=right_index, sort=sort, suffixes=suffixes,
     34                          copy=copy, indicator=indicator)
---> 35     return op.get_result()
     36 if __debug__:
     37     merge.__doc__ = _merge_doc % '\nleft : DataFrame'

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in get_result(self)
    194             self.left, self.right = self._indicator_pre_merge(self.left, self.right)
    195
--> 196         join_index, left_indexer, right_indexer = self._get_join_info()
    197
    198         ldata, rdata = self.left._data, self.right._data

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in _get_join_info(self)
    304     def _get_join_info(self):
    305         left_ax = self.left._data.axes[self.axis]
--> 306         right_ax = self.right._data.axes[self.axis]
    307
    308         if self.left_index and self.right_index:

IndexError: list index out of range

And then there's this as well:

In [10]: pd.merge(df, df.a)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-10-dba79c04ae03> in <module>()
----> 1 pd.merge(df, df.a)

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator)
     32                          right_on=right_on, left_index=left_index,
     33                          right_index=right_index, sort=sort, suffixes=suffixes,
---> 34                          copy=copy, indicator=indicator)
     35     return op.get_result()
     36 if __debug__:

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in __init__(self, left, right, how, on, left_on, right_on, axis, left_index, right_index, sort, suffixes, copy, indicator)
    188         (self.left_join_keys,
    189          self.right_join_keys,
--> 190          self.join_names) = self._get_merge_keys()
    191
    192     def get_result(self):

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in _get_merge_keys(self)
    376         left_keys, right_keys
    377         """
--> 378         self._validate_specification()
    379
    380         left_keys = []

/usr/local/lib/python3.5/site-packages/pandas/tools/merge.py in _validate_specification(self)
    465                 # use the common columns
    466                 common_cols = self.left.columns.intersection(
--> 467                     self.right.columns)
    468                 if len(common_cols) == 0:
    469                     raise MergeError('No common columns to perform merge on')

/usr/local/lib/python3.5/site-packages/pandas/core/generic.py in __getattr__(self, name)
   2393                 or name in self._metadata
   2394                 or name in self._accessors):
-> 2395             return object.__getattribute__(self, name)
   2396         else:
   2397             if name in self._info_axis:

AttributeError: 'Series' object has no attribute 'columns'

[jreback]

thanks!. yes these could use more intuitive warnings. want to do a PR?

[dan-tee]

I'd love to give it a shot.

[dan-tee]

I've opened #12112