BUG: merge_asof with non-unique on, left_on, or right_on, raises AttributeError: 'DataFrame' object has no attribute 'dtype'

[mvashishtha]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd

left = pd.DataFrame([[1, 2, 'a'], [5, 6, 'b'], [10, 11, 'c']], columns=['a', 'a', 'left_val'])
right = pd.DataFrame([[x] * 3 for x in [1, 2, 3, 6, 7]], columns=['a', 'a', 'right_val'])
print(pd.merge_asof(left, right, on="a"))
print(pd.merge_asof(left, right, left_on="a", right_on='right_val'))
print(pd.merge_asof(left, right, left_on="left_val", right_on='a'))

Issue Description

I get AttributeError: 'DataFrame' object has no attribute 'dtype' when I pass non-unique on, left_on, or right_on. Stack trace for on from code above:

stack trace

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Input In [28], in <cell line: 1>()
----> 1 print(pd.merge_asof(left, right, on="a"))

File ~/opt/anaconda3/envs/modin-dev-py38/lib/python3.8/site-packages/pandas/core/reshape/merge.py:583, in merge_asof(left, right, on, left_on, right_on, left_index, right_index, by, left_by, right_by, suffixes, tolerance, allow_exact_matches, direction)
    328 def merge_asof(
    329     left: DataFrame | Series,
    330     right: DataFrame | Series,
   (...)
    342     direction: str = "backward",
    343 ) -> DataFrame:
    344     """
    345     Perform a merge by key distance.
    346
   (...)
    581     4 2016-05-25 13:30:00.048   AAPL   98.00       100     NaN     NaN
    582     """
--> 583     op = _AsOfMerge(
    584         left,
    585         right,
    586         on=on,
    587         left_on=left_on,
    588         right_on=right_on,
    589         left_index=left_index,
    590         right_index=right_index,
    591         by=by,
    592         left_by=left_by,
    593         right_by=right_by,
    594         suffixes=suffixes,
    595         how="asof",
    596         tolerance=tolerance,
    597         allow_exact_matches=allow_exact_matches,
    598         direction=direction,
    599     )
    600     return op.get_result()

File ~/opt/anaconda3/envs/modin-dev-py38/lib/python3.8/site-packages/pandas/core/reshape/merge.py:1825, in _AsOfMerge.__init__(self, left, right, on, left_on, right_on, left_index, right_index, by, left_by, right_by, axis, suffixes, copy, fill_method, how, tolerance, allow_exact_matches, direction)
   1822 self.allow_exact_matches = allow_exact_matches
   1823 self.direction = direction
-> 1825 _OrderedMerge.__init__(
   1826     self,
   1827     left,
   1828     right,
   1829     on=on,
   1830     left_on=left_on,
   1831     right_on=right_on,
   1832     left_index=left_index,
   1833     right_index=right_index,
   1834     axis=axis,
   1835     how=how,
   1836     suffixes=suffixes,
   1837     fill_method=fill_method,
   1838 )

File ~/opt/anaconda3/envs/modin-dev-py38/lib/python3.8/site-packages/pandas/core/reshape/merge.py:1726, in _OrderedMerge.__init__(self, left, right, on, left_on, right_on, left_index, right_index, axis, suffixes, fill_method, how)
   1710 def __init__(
   1711     self,
   1712     left: DataFrame | Series,
   (...)
   1722     how: str = "outer",
   1723 ) -> None:
   1725     self.fill_method = fill_method
-> 1726     _MergeOperation.__init__(
   1727         self,
   1728         left,
   1729         right,
   1730         on=on,
   1731         left_on=left_on,
   1732         left_index=left_index,
   1733         right_index=right_index,
   1734         right_on=right_on,
   1735         axis=axis,
   1736         how=how,
   1737         suffixes=suffixes,
   1738         sort=True,  # factorize sorts
   1739     )

File ~/opt/anaconda3/envs/modin-dev-py38/lib/python3.8/site-packages/pandas/core/reshape/merge.py:685, in _MergeOperation.__init__(self, left, right, how, on, left_on, right_on, axis, left_index, right_index, sort, suffixes, indicator, validate)
    681     # stacklevel chosen to be correct when this is reached via pd.merge
    682     # (and not DataFrame.join)
    683     warnings.warn(msg, FutureWarning, stacklevel=find_stack_level())
--> 685 self.left_on, self.right_on = self._validate_left_right_on(left_on, right_on)
    687 cross_col = None
    688 if self.how == "cross":

File ~/opt/anaconda3/envs/modin-dev-py38/lib/python3.8/site-packages/pandas/core/reshape/merge.py:1873, in _AsOfMerge._validate_left_right_on(self, left_on, right_on)
   1870         lo_dtype = left_on_0.dtype
   1871     else:
   1872         lo_dtype = (
-> 1873             self.left[left_on_0].dtype
   1874             if left_on_0 in self.left.columns
   1875             else self.left.index.get_level_values(left_on_0)
   1876         )
   1877 else:
   1878     lo_dtype = self.left.index.dtype

File ~/opt/anaconda3/envs/modin-dev-py38/lib/python3.8/site-packages/pandas/core/generic.py:5902, in NDFrame.__getattr__(self, name)
   5895 if (
   5896     name not in self._internal_names_set
   5897     and name not in self._metadata
   5898     and name not in self._accessors
   5899     and self._info_axis._can_hold_identifiers_and_holds_name(name)
   5900 ):
   5901     return self[name]
-> 5902 return object.__getattribute__(self, name)

AttributeError: 'DataFrame' object has no attribute 'dtype'

Expected Behavior

I should get something more informative like the error for non-unique by, which is MergeError: Data columns not unique: Index(['a'], dtype='object')

Installed Versions

INSTALLED VERSIONS

commit : 8dab54d
python : 3.8.13.final.0
python-bits : 64
OS : Darwin
OS-release : 21.5.0
Version : Darwin Kernel Version 21.5.0: Tue Apr 26 21:08:22 PDT 2022; root:xnu-8020.121.3~4/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.5.2
numpy : 1.23.1
pytz : 2022.1
dateutil : 2.8.2
setuptools : 61.2.0
pip : 22.1.2
Cython : None
pytest : 7.1.2
hypothesis : None
sphinx : 5.1.1
blosc : None
feather : 0.4.1
xlsxwriter : None
lxml.etree : 4.9.1
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.4.0
pandas_datareader: None
bs4 : 4.11.1
bottleneck : None
brotli : None
fastparquet : 2022.11.0
fsspec : 2022.7.1
gcsfs : None
matplotlib : 3.5.2
numba : None
numexpr : 2.8.3
odfpy : None
openpyxl : 3.0.10
pandas_gbq : 0.17.7
pyarrow : 8.0.0
pyreadstat : None
pyxlsb : None
s3fs : 2022.7.1
scipy : 1.9.0
snappy : None
sqlalchemy : 1.4.39
tables : 3.7.0
tabulate : 0.9.0
xarray : 2022.6.0
xlrd : 2.0.1
xlwt : None
zstandard : None
tzdata : None