您基本上可以通过以下方式实现此目的：

df_melted = df.melt(id_vars=["Year"], value_vars=months, var_name="Month", value_name="CPI")

这将看起来像：

Year Month  CPI
0    1984   Jan 0.48
1    1985   Jan 0.47
2    1986   Jan 0.64
3    1987   Jan 0.52
4    1988   Jan 1.00

从那里你可以很容易地让它看起来像你想要的。例如：

# helpful
months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
month_map = dict(zip(months, [i for i in range(1,13)]))

# get it to look like your ideal outcome
df_melted = df_melted.replace({"Month": month_map})
ym = [f"{y}-{m:02d}" for y, m in df_melted[["Year", "Month"]].values]
df_melted.insert(0, "Year-Month", ym)
df_melted.drop(["Year", "Month"], axis=1, inplace=True)

现在它看起来就像你想要的结果。