BUG: Unclear error message assign a column with an empty DataFrame · Issue #55956 · pandas-dev/pandas (original) (raw)

Edit: This turned out to be an issue with the error message when setting a single column using a DataFrame with no columns; see #55956 (comment) - rhshadach

Pandas version checks

Reproducible Example

import pandas as pd df = pd.DataFrame() df['test1'] = df.apply(lambda x: x['test'] + "_",axis=1)

Issue Description

I was expecting to see KeyError 'test' as an error, but I get an error as follows:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[4], line 1
----> 1 df['test1'] = df.apply(lambda x: x['test'] + "_",axis=1)

File ~\AppData\Local\anaconda3\lib\site-packages\pandas\core\frame.py:4081, in DataFrame.__setitem__(self, key, value)
   4079     self._setitem_array(key, value)
   4080 elif isinstance(value, DataFrame):
-> 4081     self._set_item_frame_value(key, value)
   4082 elif (
   4083     is_list_like(value)
   4084     and not self.columns.is_unique
   4085     and 1 < len(self.columns.get_indexer_for([key])) == len(value)
   4086 ):
   4087     # Column to set is duplicated
   4088     self._setitem_array([key], value)

File ~\AppData\Local\anaconda3\lib\site-packages\pandas\core\frame.py:4239, in DataFrame._set_item_frame_value(self, key, value)
   4236     return self.isetitem(locs, value)
   4238 if len(value.columns) != 1:
-> 4239     raise ValueError(
   4240         "Cannot set a DataFrame with multiple columns to the single "
   4241         f"column {key}"
   4242     )
   4244 self[key] = value[value.columns[0]]

ValueError: Cannot set a DataFrame with multiple columns to the single column test1

Expected Behavior

I was expecting to see KeyError 'test' in the error traceback

Installed Versions

Python 3.10.9, pandas 2.1.3, numpy 1.24.3 - Windows 10

ImportError Traceback (most recent call last)
Cell In[6], line 1
----> 1 pd.show_versions()

File ~\AppData\Local\anaconda3\lib\site-packages\pandas\util_print_versions.py:141, in show_versions(as_json)
104 """
105 Provide useful information, important for bug reports.
106
(...)
138 ...
139 """
140 sys_info = _get_sys_info()
--> 141 deps = _get_dependency_info()
143 if as_json:
144 j = {"system": sys_info, "dependencies": deps}

File ~\AppData\Local\anaconda3\lib\site-packages\pandas\util_print_versions.py:98, in _get_dependency_info()
96 result: dict[str, JSONSerializable] = {}
97 for modname in deps:
---> 98 mod = import_optional_dependency(modname, errors="ignore")
99 result[modname] = get_version(mod) if mod else None
100 return result

File ~\AppData\Local\anaconda3\lib\site-packages\pandas\compat_optional.py:147, in import_optional_dependency(name, extra, errors, min_version)
145 minimum_version = min_version if min_version is not None else VERSIONS.get(parent)
146 if minimum_version:
--> 147 version = get_version(module_to_get)
148 if version and Version(version) < Version(minimum_version):
149 msg = (
150 f"Pandas requires version '{minimum_version}' or newer of '{parent}' "
151 f"(version '{version}' currently installed)."
152 )

File ~\AppData\Local\anaconda3\lib\site-packages\pandas\compat_optional.py:74, in get_version(module)
71 version = getattr(module, "version", None)
73 if version is None:
---> 74 raise ImportError(f"Can't determine version for {module.name}")
75 if module.name == "psycopg2":
76 # psycopg2 appends " (dt dec pq3 ext lo64)" to it's version
77 version = version.split()[0]

ImportError: Can't determine version for xlrd