Question

很抱歉您提出一个幼稚的问题，但此刻让我发疯。我有一个数据框df1，并使用它来创建新的数据框df2，如下所示：

import pandas as pd
def NewDF(df):
   df['sum']=df['a']+df['b']
   return df
df1 =pd.DataFrame({'a':[1,2,3],'b':[4,5,6]})
print(df1)
df2 =NewDF(df1)
print(df1)

给出

   a  b
0  1  4
1  2  5
2  3  6

   a  b  sum
0  1  4    5
1  2  5    7
2  3  6    9

为什么我失去df1形状并获得第三列？如何避免这种情况？

Answer 1

DataFrames是mutable，因此您应该将副本明确传递给函数，或者让函数的第一步复制输入。否则，就像列表一样，您对函数所做的任何修改也将应用于原始列表。

您的选择是：

def NewDF(df):
   df = df.copy()
   df['sum']=df['a']+df['b']
   return df

df2 = NewDF(df1)

或

df2 = NewDF(df1.copy())

在这里我们可以看到原始实现中的所有内容都指向同一对象

import pandas as pd
def NewDF(df):
    print(id(df))
    df['sum']=df['a']+df['b']
    print(id(df))
    return df

df1 =pd.DataFrame({'a':[1,2,3],'b':[4,5,6]})

print(id(df1))
#2242099787480

df2 = NewDF(df1)
#2242099787480
#2242099787480

print(id(df2))
#2242099787480

Answer 2

获得的第三列是“索引”列，每个pandas DataFrame都将始终保留一个索引，但是您可以选择是否在输出中不使用它。

C:\Users\esin>pip install beautifulsoup4
Collecting beautifulsoup4
  Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))': /simple/beautifulsoup4/
  Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))': /simple/beautifulsoup4/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))': /simple/beautifulsoup4/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))': /simple/beautifulsoup4/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))': /simple/beautifulsoup4/
  Could not fetch URL https://pypi.org/simple/beautifulsoup4/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/beautifulsoup4/ (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))) - skipping
  Could not find a version that satisfies the requirement beautifulsoup4 (from versions: )
No matching distribution found for beautifulsoup4
Could not fetch URL https://pypi.org/simple/pip/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/pip/ (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1056)'))) - skipping

将输出显示为

import pandas as pd
def NewDF(df):
   df['sum']=df['a']+df['b']
   return df
df1 =pd.DataFrame({'a':[1,2,3],'b':[4,5,6]})
print(df1.to_string(index=False))
df2 =NewDF(df1)
print(df1.to_string(index = False))

现在您可能有一个问题，为什么索引存在，索引实际上是一个支持后备的哈希表，它可以提高速度，并且在多个上下文中是非常可取的功能，如果这只是一个问题，那么就足够了，但是如果您想了解有关熊猫的更多信息，我建议您研究索引，您可以从这里https://stackoverflow.com/a/27238758/10953776

开始

构建新的数据框并保留旧的数据框？

2 个答案: