Python DataFrame - ValueError：解包的值太多（预期 2）

Question

Python 3.8

我是 Python 的新手，我需要在 2020 年 11 月 12 日星期五之前交付这个项目。 我有一个 DataFrame，我必须打印 name_list 中保存的列的“价格”键的平均值和标准差。

这是我写的代码：

#Grouping by company, fuel type, aspiration, door number
#Creating an empty DataFrame
result = pd.DataFrame()
#Saving all the prices
values = df['price'].unique()
name_list = ['CarName', 'fueltype', 'aspiration', 'doornumber']
#Cycling for each column and saving the mean and standard deviation in the empty dataframe
for name, value in name_list, values:
    result["Mean Price = ", value] = df[df.price == value][name].mean()
    result["Standard Deviation = ", value] = df[df.price == value][name].std()

#result = result.dropna()

#Saving mean price and standard deviation to a csv file
result.to_csv("MeanPrice_StdDeviation.csv", index=False)

当我运行代码时，我得到了错误： ValueError：要解压的值太多（预期为 2）

我在网上读到你可以使用 items 方法，但是如果我尝试这样做，它就不起作用，因为我得到这个错误： AttributeError：“numpy.ndarray”对象没有属性“items”

我该如何解决这个问题？

编辑： 这是我尝试嵌套 for 循环时收到的完整错误消息。 我这部分代码的行数从 38 到 53。

Traceback (most recent call last):

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 1303, in _ensure_numeric
    x = float(x)

ValueError: could not convert string to float: 'alfa-romero giulia'


During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 1307, in _ensure_numeric
    x = complex(x)

ValueError: complex() arg is a malformed string


During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "c:\users\mazza\documents\informatica\progetto_esame_peruzzini_mazzanti.py", line 48, in <module>
    result["Mean Price = ", value] = df[df.price == value][name].mean()

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\generic.py", line 11214, in stat_func
    return self._reduce(

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\series.py", line 3891, in _reduce
    return op(delegate, skipna=skipna, **kwds)

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 69, in _f
    return f(*args, **kwargs)

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 125, in f
    result = alt(values, axis=axis, skipna=skipna, **kwds)

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 542, in nanmean
    the_sum = _ensure_numeric(values.sum(axis, dtype=dtype_sum))

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 1310, in _ensure_numeric
    raise TypeError(f"Could not convert {x} to numeric")

TypeError: Could not convert alfa-romero giulia to numeric


debugfile('C:/Users/mazza/Documents/Informatica/progetto_esame_peruzzini_mazzanti.py', wdir='C:/Users/mazza/Documents/Informatica')

Traceback (most recent call last):

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 1303, in _ensure_numeric
    x = float(x)

ValueError: could not convert string to float: 'alfa-romero giulia'


During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 1307, in _ensure_numeric
    x = complex(x)

ValueError: complex() arg is a malformed string


During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "c:\users\mazza\documents\informatica\progetto_esame_peruzzini_mazzanti.py", line 47, in <module>
    result["Mean Price = ", value] = df[df.price == value][name].mean()

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\generic.py", line 11214, in stat_func
    return self._reduce(

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\series.py", line 3891, in _reduce
    return op(delegate, skipna=skipna, **kwds)

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 69, in _f
    return f(*args, **kwargs)

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 125, in f
    result = alt(values, axis=axis, skipna=skipna, **kwds)

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 542, in nanmean
    the_sum = _ensure_numeric(values.sum(axis, dtype=dtype_sum))

  File "C:\Users\mazza\anaconda3\lib\site-packages\pandas\core\nanops.py", line 1310, in _ensure_numeric
    raise TypeError(f"Could not convert {x} to numeric")

TypeError: Could not convert alfa-romero giulia to numeric

Answer 1

您遇到此问题是因为 name_list 和 values 的长度不等。使用两个 for 循环（嵌套）尝试以下代码：

#Grouping by company, fuel type, aspiration, door number
#Creating an empty DataFrame
result = pd.DataFrame()
#Saving all the prices
values = df['price'].unique()
name_list = ['CarName', 'fueltype', 'aspiration', 'doornumber']
#Cycling for each column and saving the mean and standard deviation in the empty dataframe   
for name in name_list:
    for value in values:
        result["Mean Price = ", value] = df[df.price == value][name].mean()
        result["Standard Deviation = ", value] = df[df.price == value][name].std()

#result = result.dropna()

#Saving mean price and standard deviation to a csv file
result.to_csv("MeanPrice_StdDeviation.csv", index=False)

编辑以下 OP 的 cmets：

#Grouping by company, fuel type, aspiration, door number
#Creating an empty DataFrame
result = pd.DataFrame()
#Saving all the prices
values = df['price'].unique()
name_list = ['CarName', 'fueltype', 'aspiration', 'doornumber']
#Cycling for each column and saving the mean and standard deviation in the empty dataframe   
for column in name_list:    
    print(df.groupby([column])['price'].mean().reset_index())
    print(df.groupby([column])['price'].std().reset_index())