我将数据从csv文件导入到Pandas数据框中。我从字符串中删除了非数字值，即所有值前面的“$”。然后我将列转换为float数据类型。转换后我运行print（df.dtypes），它将所有列显示为float 64。在print语句后，我试图从另一列中减去一列，但得到一个错误消息：

line 23, in <module>
    Price_Diff = df["HTB_Price" - "McMaster_Price"]
TypeError: unsupported operand type(s) for -: 'str' and 'str'

下面是我的代码

import pandas as pd
import matplotlib.pyplot as mp
import numpy as np

# Reads the csv and create a dataframe titled "df"
df = pd.read_csv('Example Price Dataset.csv', sep='\s*,\s*', engine='python')

# Removes the "$" from all columns using a left strip
df['HTB_Price'] = df['HTB_Price'].map(lambda x: x.lstrip('$'))
df['McMaster_Price'] = df['McMaster_Price'].map(lambda x: x.lstrip('$'))
df['Motion_Price'] = df['Motion_Price'].map(lambda x: x.lstrip('$'))
df['MRO_Price'] = df['MRO_Price'].map(lambda x: x.lstrip('$'))

# Converts each column to a float datatype instead of a string
df["HTB_Price"] = df["HTB_Price"].astype(float)
df["McMaster_Price"] = df["McMaster_Price"].astype(float)
df["Motion_Price"] = df["Motion_Price"].astype(float)
df["MRO_Price"] = df["MRO_Price"].astype(float)
print(df.dtypes)

#
Price_Diff = df["HTB_Price" - "McMaster_Price"]

# Prints the dataframe
# print(df.dtypes)

错误在Price_Diff行上，我不知道为什么它会抛出一个关于不能相互减去字符串的错误，而就在这一行之前，我正在检查数据类型，它说它们都是浮点型。
我希望每列中的值相减并放入变量Price_Diff中