I created an empty dataframe and tried adding one string value to a column but it is showing empty
import pandas as pd
df = pd.DataFrame()
df['Man'] = "manish"
print(df)
when i am running above code i am getting output as:
Empty DataFrame
Columns: [Man]
Index: []
While when i am running below code
df['Man'] = ['manish']
print(df)
i am getting correct output which i expected
Man
0 manish
Can anyone explain me why this is happening ?
It seems, by looking at the code of __setitem__ that it expects a list like value, as written in the function _ensure_valid_index:
def _ensure_valid_index(self, value):
"""
Ensure that if we don't have an index, that we can create one from the
passed value.
"""
# GH5632, make sure that we are a Series convertible
if not len(self.index) and is_list_like(value):
try:
value = Series(value)
except (ValueError, NotImplementedError, TypeError):
raise ValueError(
"Cannot set a frame with no defined index "
"and a value that cannot be converted to a "
"Series"
)
self._data = self._data.reindex_axis(
value.index.copy(), axis=1, fill_value=np.nan
)
So if the len of the index is zero (as in your case) it expects a list like value, which a string is not, to convert to a Series an use the index from there. The function _ensure_valid_index is called inside set_item.
df['Man'] == "manish"is comparison.==(two equal signs) is a test for equality.=is an assign (this is what you want here)[]you indicate there's a length of 1. IMO it's odd thatpandasallows this to work becauselen(df) == 0andlen(['manish']) == 1. In all cases where the index has a positive size and they mismatch you'd receiveValueError: Length of values does not match length of indexdf['Man'] == "manish"i added by mistake , even withdf['Man'] = "manish"same issue persists