Why Does My Pandas Lambda Return Only One Column? The Fix Explained
This article walks through a pandas DataFrame issue where a lambda function intended to mimic Excel's IF logic only returns one column, explains why the else clause fails, and provides the corrected code using isna for proper conditional handling.
Hello, I am PiPi.
1. Introduction
In a Python community I was asked about a Pandas data‑analysis problem involving an Excel‑style IF function.
Original code:
df['科目修正'] = df.apply(lambda x:x['科目1'] if x['科目1']!=None else x['科目'],axis=1)The problem: the else part never executes, so only the value from 科目1 is returned.
2. Solution
Because missing values in Pandas are represented by NaN, use isna() (or pd.isna()) to test for them:
df['科目修正'] = df.apply(lambda x: x['科目'] if pd.isna(x['科目1']) else x['科目1'], axis=1)3. Summary
The article presents a concrete Pandas data‑handling issue and demonstrates how to correctly implement Excel‑like conditional logic using isna() within a lambda applied to a DataFrame.
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