在 pandas 系列中查找值 - Python3

Question

我遇到了这个非常烦人的问题（我是 python 的新手）

df=pd.DataFrame[{'col1':['1','2','3','4']}]

col1=df['col1']

为什么 col1[1] in col1 return False?

Answer 1

检查值使用 boolean indexing:

#get value where index is 1
print (col1[1])
2 
#more common with loc
print (col1.loc[1])
2

print (col1 == '2')
0    False
1     True
2    False
3    False
Name: col1, dtype: bool

如果需要获取行：

print (col1[col1 == '2'])
1    2
Name: col1, dtype: object

用or检查多个值：

print (col1.isin(['2', '4']))
0    False
1     True
2    False
3     True
Name: col1, dtype: bool 

print (col1[col1.isin(['2', '4'])])
1    2
3    4
Name: col1, dtype: object

还有一些关于 in 的测试会员资格 docs:

Using the Python in operator on a Series tests for membership in the index, not membership among the values.

If this behavior is surprising, keep in mind that using in on a Python dictionary tests keys, not values, and Series are dict-like. To test for membership in the values, use the method isin():

For DataFrames, likewise, in applies to the column axis, testing for membership in the list of column names.

#1 is in index
print (1 in col1)
True

#5 is not in index
print (5 in col1)
False

#string 2 is not in index
print ('2' in col1)
False

#number 2 is in index
print (2 in col1)
True

您尝试在索引值中查找字符串 2：

print (col1[1])
2

print (type(col1[1]))
<class 'str'>

print (col1[1] in col1)
False

Answer 2

我可能遗漏了一些东西，这是多年后的事了，但是当我读到这个问题时，您正试图让 in 关键字用于您的熊猫系列？所以可能想做：

col1[1] in col1.values

因为上面说了，pandas是通过索引来查找的，你需要特别要求它查看系列的值，而不是索引。

在 pandas 系列中查找值 - Python3

finding values in pandas series - Python3

series

python-3.x

pandas