Fix: KeyError on pandas merge with mismatched dtypes

What happened

df1.merge(df2, on="user_id")
# KeyError: 'user_id'

Both DataFrames have user_id. So what’s wrong?

Root cause

dtypes mismatch. df1.user_id is int64, df2.user_id is object (string).

Fix

df2["user_id"] = df2["user_id"].astype(int)
df1.merge(df2, on="user_id")

Always check df.dtypes before merge.