dataframe pandas pivot python

ValueError when trying to have multi-index in DataFrame.pivot

I have read pandas: how to run a pivot with a multi-index? but it could not solve my problem.

Given the data frame below:

import pandas as pd
df = pd.DataFrame({
"date": ["20180920"] * 6,
"id": ["A123456789"] * 6,
"test": ["a", "b", "c", "d", "e", "f"],
"result": [70, 90, 110, "(-)", "(+)", 0.3],
"ref": ["< 90", "70 - 100", "100 - 120", "(-)", "(-)", "< 1"]

I’d like to spread the test column, use the values in result, and ignore ref. In other words, the desired output is like:

       date          id      a   b    c    d    e    f
0 20180920 A123456789 70 90 110 (-) (+) 0.3

So I tried df.pivot(index=["date", "id"], columns="test", values="result"), but it failed with ValueError: Length of passed values is 6, index implies 2. I think it is related to “If an array is passed, it must be the same length as the data.” in pivot_table documentation, but I just don’t understand what it means. Can someone elaborate that please?

BTW, I finally get my desired output by df.drop(columns="ref").set_index(["date", "id", "test"]).unstack(level=2). Is it the only correct way?