Python3快速入门（十五）——Pandas数据处理
一、函数应用
1、函数应用简介
如果要将自定义函数或其它库函数应用于Pandas对象，有三种使用方式。pipe()将函数用于表格，apply()将函数用于行或列，applymap()将函数用于元素。
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2、表格函数应用
可以通过将函数对象和参数作为pipe函数的参数来执行自定义操作，会对整个DataFrame执行操作。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
def adder(x, y):
return x + y
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(5, 3),columns=['col1', 'col2', 'col3'])
print(df)
df = df.pipe(adder, 1)
print(df)
# output:
# col1 col2 col3
# 0 0.390803 0.940306 -1.300635
# 1 -0.349588 -1.290132 0.415693
# 2 -0.079585 -0.083825 0.262867
# 3 0.582377 0.171701 -1.011748
# 4 -0.466655 1.746269 1.281538
# col1 col2 col3
# 0 1.390803 1.940306 -0.300635
# 1 0.650412 -0.290132 1.415693
# 2 0.920415 0.916175 1.262867
# 3 1.582377 1.171701 -0.011748
# 4 0.533345 2.746269 2.281538
3、行、列函数应用
使用apply()函数可以沿DataFrame或Panel的轴执行应用函数，采用可选axis参数。 默认情况下，操作按列执行。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
def adder(x, y):
return x + y
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(5, 3), columns=['col1', 'col2', 'col3'])
print(df)
# 按列执行
result = df.apply(np.sum)
print(result)
# 按行执行
result = df.apply(np.sum, axis=1)
print(result)
# output:
# col1 col2 col3
# 0 -1.773775 -0.608478 0.602059
# 1 -0.208412 0.969435 -0.292108
# 2 0.776864 -0.768559 -0.389092
# 3 -2.088412 1.133090 1.006486
# 4 0.693241 1.808845 0.772191
# col1 -2.600494
# col2 2.534332
# col3 1.699536
# dtype: float64
# 0 -1.780194
# 1 0.468915
# 2 -0.380788
# 3 0.051164
# 4 3.274277
# dtype: float64
4、元素函数应用
在DataFrame的applymap()函数可以接受任何Python函数，并且返回单个值。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(5, 3), columns=['col1', 'col2', 'col3'])
print(df)
df = df.applymap(lambda x: x + 1)
print(df)
# output:
# col1 col2 col3
# 0 2.396185 -0.263581 -0.090799
# 1 1.718716 0.876074 -1.067746
# 2 -1.033945 -0.078448 1.036566
# 3 0.553849 0.251312 -0.422640
# 4 -0.896062 1.605349 -0.089430
# col1 col2 col3
# 0 3.396185 0.736419 0.909201
# 1 2.718716 1.876074 -0.067746
# 2 -0.033945 0.921552 2.036566
# 3 1.553849 1.251312 0.577360
# 4 0.103938 2.605349 0.910570
二、数据清洗
1、数据清洗简介
数据清洗是一项复杂且繁琐的工作，同时也是数据分析过程中最为重要的环节。数据清洗的目的一是通过清洗让数据可用，二是让数据变的更适合进行数据分析工作。因此，脏数据要清洗，干净数据也要清洗。在实际数据分析中，数据清洗将占用项目70%左右的时间。
2、缺失值处理
查看每一列有多少缺失值。
df.isnull().sum()
查看每一列有多少完整的数据
df.shape[0]-df.isnull().sum()
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 3), index=dates, columns=list('ABC'))
print(df)
print(df.isnull().sum())
print(df.shape[0] - df.isnull().sum())
# output:
# A B C
# 2019-01-01 1.138325 0.981597 1.359580
# 2019-01-02 -1.622074 0.812393 -0.946351
# 2019-01-03 0.049815 1.194241 0.807209
# 2019-01-04 1.500074 -0.570367 -0.328529
# 2019-01-05 0.465869 1.049651 -0.112453
# 2019-01-06 -1.399495 0.492769 1.961198
# A 0
# B 0
# C 0
# dtype: int64
# A 6
# B 6
# C 6
# dtype: int64
删除列
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
print(df)
del df['D']
# 删除第2列
df.drop(df.columns[2], axis=1, inplace=True)
# 删除B列
df.drop('B', axis=1, inplace=True)
print(df)
# output:
# A B C
# 2019-01-01 -0.703151 0.753482 -0.624376
# 2019-01-02 -0.396221 -0.832279 -1.419897
# 2019-01-03 -0.179341 -0.368501 -0.300810
# 2019-01-04 0.464156 0.117461 1.502114
# 2019-01-05 -1.022012 -1.612456 1.611377
# 2019-01-06 -0.677521 0.001020 -0.342290
# A
# 2019-01-01 -0.703151
# 2019-01-02 -0.396221
# 2019-01-03 -0.179341
# 2019-01-04 0.464156
# 2019-01-05 -1.022012
# 2019-01-06 -0.677521
删除NaN值
df.dropna(self, axis=0, how='any', thresh=None, subset=None,
inplace=False)
axis为轴，0表示对行进行操作，1表示对列进行操作。
how为操作类型，’any’表示只要出现NaN的行或列都删除，’all’表示删除整行或整列都为NaN的行或列。
thresh：NaN的阈值，达到thresh时删除。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
df.iloc[1, 3] = None
df.iloc[2, 2] = None
print(df)
print(df.dropna(axis=1))
print(df.dropna(how='any'))
# output:
# A B C D
# 2019-01-01 -0.152239 -2.315100 -0.504998 -0.987549
# 2019-01-02 -1.884801 1.046506 -1.618871 NaN
# 2019-01-03 0.976682 -1.043107 NaN 0.391338
# 2019-01-04 0.143389 0.951518 0.040632 -0.443944
# 2019-01-05 3.092766 0.787921 -2.408260 -1.111238
# 2019-01-06 -0.179249 0.573734 -0.912023 0.261517
# A B
# 2019-01-01 -0.152239 -2.315100
# 2019-01-02 -1.884801 1.046506
# 2019-01-03 0.976682 -1.043107
# 2019-01-04 0.143389 0.951518
# 2019-01-05 3.092766 0.787921
# 2019-01-06 -0.179249 0.573734
# A B C D
# 2019-01-01 -0.152239 -2.315100 -0.504998 -0.987549
# 2019-01-04 0.143389 0.951518 0.040632 -0.443944
# 2019-01-05 3.092766 0.787921 -2.408260 -1.111238
# 2019-01-06 -0.179249 0.573734 -0.912023 0.261517
填充NaN值
df.fillna(self, value=None, method=None, axis=None, inplace=False,limit=None, downcast=None, **kwargs)
value:填充的值，可以为字典，字典的key为列名称。
inplace:表示是否对源数据进行修改，默认为False。
fillna默认会返回新对象，但也可以对现有对象进行就地修改。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
df.iloc[1, 3] = None
df.iloc[2, 2] = None
print(df)
print(df.fillna({'C': 3.14, 'D': 0.0}))
# 使用指定值填充
df.fillna(value=3.14, inplace=True)
print(df)
# output:
# A B C D
# 2019-01-01 0.490727 -0.603079 0.202922 2.012060
# 2019-01-02 -0.855106 0.305557 0.851141 NaN
# 2019-01-03 -0.324215 0.629637 NaN -0.174930
# 2019-01-04 0.085996 0.173265 0.416938 -0.903989
# 2019-01-05 0.009368 0.410056 -1.297822 -2.202893
# 2019-01-06 0.021892 -0.359749 -0.608556 -0.859454
# A B C D
# 2019-01-01 0.490727 -0.603079 0.202922 2.012060
# 2019-01-02 -0.855106 0.305557 0.851141 0.000000
# 2019-01-03 -0.324215 0.629637 3.140000 -0.174930
# 2019-01-04 0.085996 0.173265 0.416938 -0.903989
# 2019-01-05 0.009368 0.410056 -1.297822 -2.202893
# 2019-01-06 0.021892 -0.359749 -0.608556 -0.859454
# A B C D
# 2019-01-01 0.490727 -0.603079 0.202922 2.012060
# 2019-01-02 -0.855106 0.305557 0.851141 3.140000
# 2019-01-03 -0.324215 0.629637 3.140000 -0.174930
# 2019-01-04 0.085996 0.173265 0.416938 -0.903989
# 2019-01-05 0.009368 0.410056 -1.297822 -2.202893
# 2019-01-06 0.021892 -0.359749 -0.608556 -0.859454
对数据进行布尔填充
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
df.iloc[1, 3] = None
df.iloc[2, 2] = None
print(df)
print(pd.isnull(df))
# output:
# A B C D
# 2019-01-01 -1.337471 0.154446 0.493862 1.278946
# 2019-01-02 2.853301 -0.151376 0.318281 NaN
# 2019-01-03 1.094465 0.059063 NaN 0.216805
# 2019-01-04 -0.983091 -1.052905 0.416604 -1.431156
# 2019-01-05 -1.421142 1.015465 -1.851315 -0.680514
# 2019-01-06 0.224378 -0.636699 -0.749040 -0.728368
# A B C D
# 2019-01-01 False False False False
# 2019-01-02 False False False True
# 2019-01-03 False False True False
# 2019-01-04 False False False False
# 2019-01-05 False False False False
# 2019-01-06 False False False False
3、行和列处理
通过字典键可以进行列选择，获取DataFrame中的一列数据。
生成DataFrame时指定index和columns
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20130101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
print(df)
# output:
# A B C D
# 2013-01-01 1.116914 -0.221035 -0.577299 -0.328831
# 2013-01-02 1.764656 1.462838 -0.360678 1.176134
# 2013-01-03 0.144396 -0.594359 -0.548543 1.281829
# 2013-01-04 0.632378 0.895123 -0.757924 -1.325917
# 2013-01-05 0.219125 -1.247446 0.335363 -0.676052
# 2013-01-06 0.963715 -0.131331 0.326482 -0.718461
index和columns也可以在DataFrame创建后指定
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20130101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
print(df)
df.index = pd.date_range('20130201', periods=df.shape[0])
df.columns = list('abcd')
print(df)
df.index = pd.date_range('20130301', periods=len(df))
df.columns = list('ABCD')
print(df)
# output:
# A B C D
# 2013-01-01 1.588442 1.548420 0.132539 0.410512
# 2013-01-02 0.200415 1.515354 2.275575 -1.533603
# 2013-01-03 0.838294 0.067409 -1.157181 0.401973
# 2013-01-04 0.551363 -0.749296 0.343762 -1.558969
# 2013-01-05 -0.799507 -1.343379 -0.006312 1.091014
# 2013-01-06 0.012188 -0.382384 0.280008 -2.333430
# a b c d
# 2013-02-01 1.588442 1.548420 0.132539 0.410512
# 2013-02-02 0.200415 1.515354 2.275575 -1.533603
# 2013-02-03 0.838294 0.067409 -1.157181 0.401973
# 2013-02-04 0.551363 -0.749296 0.343762 -1.558969
# 2013-02-05 -0.799507 -1.343379 -0.006312 1.091014
# 2013-02-06 0.012188 -0.382384 0.280008 -2.333430
# A B C D
# 2013-03-01 1.588442 1.548420 0.132539 0.410512
# 2013-03-02 0.200415 1.515354 2.275575 -1.533603
# 2013-03-03 0.838294 0.067409 -1.157181 0.401973
# 2013-03-04 0.551363 -0.749296 0.343762 -1.558969
# 2013-03-05 -0.799507 -1.343379 -0.006312 1.091014
# 2013-03-06 0.012188 -0.382384 0.280008 -2.333430
可以指定某一列为index
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20130101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), columns=list('ABCD'))
df['date'] = dates
print(df)
df = df.set_index('date', drop=True)
print(df)
# output:
# A B C D date
# 0 0.910416 -0.378195 0.332562 -0.194766 2013-01-01
# 1 0.533733 0.888629 -0.358143 1.583278 2013-01-02
# 2 0.482362 -0.905558 1.045753 -0.874653 2013-01-03
# 3 0.901622 -0.535862 -0.439763 -0.640594 2013-01-04
# 4 -1.273577 -0.746785 1.448309 -0.368285 2013-01-05
# 5 0.191289 -1.246213 0.184757 -1.143074 2013-01-06
# A B C D
# date
# 2013-01-01 0.910416 -0.378195 0.332562 -0.194766
# 2013-01-02 0.533733 0.888629 -0.358143 1.583278
# 2013-01-03 0.482362 -0.905558 1.045753 -0.874653
# 2013-01-04 0.901622 -0.535862 -0.439763 -0.640594
# 2013-01-05 -1.273577 -0.746785 1.448309 -0.368285
# 2013-01-06 0.191289 -1.246213 0.184757 -1.143074
在原有DataFrame的基础上，可以创建一个新的DataFrame，或者将原有DataFrame按行进行汇总统计创建一个新的DataFrame。
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20130101', periods=6)
df = pd.DataFrame(np.random.randn(6, 3), index=dates, columns=list('ABC'))
print(df)
df1 = pd.DataFrame()
df1['min'] = df.min()
df1['max'] = df.max()
df1['std'] = df.std()
print(df1)
df['min'] = df.min(axis=1)
df['max'] = df.max(axis=1)
df['std'] = df.std(axis=1)
print(df)
# output:
# A B C
# 2013-01-01 0.901073 1.706925 -0.503194
# 2013-01-02 0.379870 0.729674 0.579337
# 2013-01-03 -1.285323 -0.665951 -0.161148
# 2013-01-04 -0.714282 0.423376 0.586061
# 2013-01-05 -0.895171 -0.413328 0.485803
# 2013-01-06 1.926472 -0.718467 1.113522
# min max std
# A -1.285323 1.926472 1.234084
# B -0.718467 1.706925 0.955797
# C -0.503194 1.113522 0.582913
# A B C min max std
# 2013-01-01 0.901073 1.706925 -0.503194 -0.503194 1.706925 1.113132
# 2013-01-02 0.379870 0.729674 0.579337 0.379870 0.729674 0.175247
# 2013-01-03 -1.285323 -0.665951 -0.161148 -1.285323 -0.161148 0.562671
# 2013-01-04 -0.714282 0.423376 0.586061 -0.714282 0.586061 0.685749
# 2013-01-05 -0.895171 -0.413328 0.485803 -0.895171 0.485803 0.696763
# 2013-01-06 1.926472 -0.718467 1.113522 -0.718467 1.926472 1.341957
axis=0，对DataFrame的每一列数据进行统计运算，得到一行。axis=0，对DataFrame的每一行数据进行统计运算，得到一列。
DataFrame可以修改index和columns。
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20130101', periods=6)
df = pd.DataFrame(np.random.randn(6, 3), index=dates, columns=list('ABC'))
print(df)
df = df.rename(index=lambda x: x + 5, columns={'A': 'newA', 'B': 'newB'})
print(df)
# output:
# A B C
# 2013-01-01 0.834910 0.652175 0.537611
# 2013-01-02 1.083902 0.836208 -1.466876
# 2013-01-03 -0.044256 0.932547 1.843682
# 2013-01-04 1.610113 -0.705734 -0.145042
# 2013-01-05 1.114897 0.273569 -0.047725
# 2013-01-06 -0.541942 -0.112752 1.644338
# newA newB C
# 2013-01-06 0.834910 0.652175 0.537611
# 2013-01-07 1.083902 0.836208 -1.466876
# 2013-01-08 -0.044256 0.932547 1.843682
# 2013-01-09 1.610113 -0.705734 -0.145042
# 2013-01-10 1.114897 0.273569 -0.047725
# 2013-01-11 -0.541942 -0.112752 1.644338
列数据的单位统一
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
df['D'] = [10000, 34000, 60000, 34000, 56000, 80000]
print(df)
for i in range(len(df['D'])):
weight = float(df.iloc[i, 3]) / 10000
df.iloc[i, 3] = '{}万'.format(weight)
print(df)
# output:
# A B C D
# 2019-01-01 -0.889533 -0.411451 0.563969 10000
# 2019-01-02 -0.573239 0.264805 -0.058530 34000
# 2019-01-03 1.224993 -1.815338 -2.075301 60000
# 2019-01-04 0.266483 1.841926 -0.759681 34000
# 2019-01-05 -0.167595 0.432617 0.533577 56000
# 2019-01-06 -0.973877 0.700821 1.093101 80000
# A B C D
# 2019-01-01 -0.889533 -0.411451 0.563969 1.0万
# 2019-01-02 -0.573239 0.264805 -0.058530 3.4万
# 2019-01-03 1.224993 -1.815338 -2.075301 6.0万
# 2019-01-04 0.266483 1.841926 -0.759681 3.4万
# 2019-01-05 -0.167595 0.432617 0.533577 5.6万
# 2019-01-06 -0.973877 0.700821 1.093101 8.0万
4、重复值删除
df.duplicated(self, subset=None, keep='first')
检查DataFrame是否有重复数据。
subset：子集，列标签或列标签的序列
keep：可选值为first，last，False，first表示保留第一个出现的值，last表示保留最后一个出现的值，False表示保留所有的值。
df.drop_duplicates(self, subset=None, keep='first', inplace=False)
删除DataFrame的重复数据。
subset：子集，列标签或列标签的序列
keep：可选值为first，last，False，first表示保留第一个出现的值，last表示保留最后一个出现的值，False表示保留所有的值。
inplace：值为True表示修改源数据，值为False表示不修改源数据
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [['Alex', np.nan, 80], ['Bob', 25, 90], ['Bob', 25, 90]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
# 使用bool过滤，取出重复的值
print(df[df.duplicated(keep=False)])
# 删除重复值，修改源数据
df.drop_duplicates(keep='last', inplace=True)
print(df)
# output:
# Name Age Score
# 0 Alex NaN 80
# 1 Bob 25.0 90
# 2 Bob 25.0 90
# Name Age Score
# 1 Bob 25.0 90
# 2 Bob 25.0 90
# Name Age Score
# 0 Alex NaN 80
# 2 Bob 25.0 90
5、异常值处理
异常值分为两种，一种是非法数据，如数字列的中间夹杂着一些汉字或者是符号；第二种是异常数据，异乎寻常的大数值或者是小数值。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
def swap(x):
if type(x) == str:
if x[-1] == '岁':
x = int(x[:-1])
elif x[-1] == '分':
x = int(x[:-1])
return x
if __name__ == "__main__":
data = [['Alex', np.nan, '89分'], ['Bob', '25岁', '90分'], ['Bob', '28岁', '90分']]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
df = df.applymap(swap)
print(df)
# output:
# Name Age Score
# 0 Alex NaN 89分
# 1 Bob 25岁 90分
# 2 Bob 28岁 90分
# Name Age Score
# 0 Alex NaN 89
# 1 Bob 25.0 90
# 2 Bob 28.0 90
6、数据格式清洗
清除字段字符的前后空格
df[‘city’]=df[‘city’].map(str.strip)
将字段进行大小写转换：
df[‘city’]=df[‘city’].str.lower()
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [['Alex', np.nan, 80], [' Bob ', 25, 90], [' Bob', 25, 90]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
# 清除字符串前后空格
print(df['Name'].map(str.strip))
# 大小写转换
print(df['Name'].str.lower())
# output:
# Name Age Score
# 0 Alex NaN 80
# 1 Bob 25.0 90
# 2 Bob 25.0 90
# 0 Alex
# 1 Bob
# 2 Bob
# Name: Name, dtype: object
# 0 alex
# 1 bob
# 2 bob
# Name: Name, dtype: object
更改列的数据类型：
df[‘price’].astype(‘int’)
7、数据替换
df[‘city’].replace(‘sh’, ‘shanghai’)
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [['Alex', np.nan, 80], ['Bob', 25, 90], ['Bob', 25, 90]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df['Name'].replace('Bob', 'Bauer'))
# output:
# Name Age Score
# 0 Alex NaN 80
# 1 Bob 25.0 90
# 2 Bob 25.0 90
# 0 Alex
# 1 Bauer
# 2 Bauer
# Name: Name, dtype: object
替换时，字符串前后不能有空格存在，必须严格匹配。
三、数据处理
1、排序
（1）按标签排序
sort_index(self, axis=0, level=None, ascending=True, inplace=False,
kind='quicksort', na_position='last', sort_remaining=True,
by=None)
使用sort_index()函数，通过传递axis参数和排序顺序，可以对DataFrame进行排序。 默认情况下，按照升序对行标签进行排序。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(4, 3), index=['rank2', 'rank1', 'rank4', 'rank3'], columns=['col1', 'col2', 'col3'])
print(df)
df = df.sort_index()
print(df)
# output:
# col1 col2 col3
# rank2 -0.627700 -0.361006 -1.126366
# rank1 -1.997538 1.569461 0.454773
# rank4 -0.598688 1.348594 0.777791
# rank3 -0.190794 -1.209312 0.830699
# col1 col2 col3
# rank1 -1.997538 1.569461 0.454773
# rank2 -0.627700 -0.361006 -1.126366
# rank3 -0.190794 -1.209312 0.830699
# rank4 -0.598688 1.348594 0.777791
通过将布尔值传递给升序参数ascending，可以控制排序顺序；通过传递axis参数值为1，可以对列标签进行排序。 默认情况下，axis = 0，对行标签进行排序。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(4, 3), index=['rank2', 'rank1', 'rank4', 'rank3'], columns=['col3', 'col2', 'col1'])
print(df)
# 按列标签进行排序
df = df.sort_index(ascending=True, axis=1)
print(df)
# output:
# col3 col2 col1
# rank2 -0.715319 -0.245760 -1.282737
# rank1 0.046705 -0.202133 0.185576
# rank4 -1.608270 -0.491281 0.047686
# rank3 -1.013456 -0.020197 1.184151
# col1 col2 col3
# rank2 -1.282737 -0.245760 -0.715319
# rank1 0.185576 -0.202133 0.046705
# rank4 0.047686 -0.491281 -1.608270
# rank3 1.184151 -0.020197 -1.013456
（2）按值排序
sort_values(self, by, axis=0, ascending=True, inplace=False,
kind='quicksort', na_position='last')
使用sort_values函数可以按值排序，接收一个by参数，使用DataFrame的列名称作为值，根据某列进行排序。by可以是列名称的列表。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(4, 3), index=['rank2', 'rank1', 'rank4', 'rank3'], columns=['col3', 'col2', 'col1'])
print(df)
df = df.sort_values(by="col2")
print(df)
# output:
# col3 col2 col1
# rank2 -0.706054 -2.135880 1.066836
# rank1 0.290660 -2.214451 -1.724394
# rank4 1.211874 0.475177 -0.711855
# rank3 -0.253331 1.211301 -0.208633
# col3 col2 col1
# rank1 0.290660 -2.214451 -1.724394
# rank2 -0.706054 -2.135880 1.066836
# rank4 1.211874 0.475177 -0.711855
# rank3 -0.253331 1.211301 -0.208633
sort_values()提供mergesort，heapsort和quicksort三种排序算法，mergesort是唯一的稳定排序算法，通过参数kind进行传递。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(4, 3), index=['rank2', 'rank1', 'rank4', 'rank3'], columns=['col3', 'col2', 'col1'])
print(df)
df = df.sort_values(by="col2", kind='mergesort')
print(df)
# output:
# col3 col2 col1
# rank2 -0.243768 -0.344846 0.535481
# rank1 -1.491950 0.690749 -2.023808
# rank4 -0.656292 -0.704788 0.655129
# rank3 0.468007 -0.250702 0.079670
# col3 col2 col1
# rank4 -0.656292 -0.704788 0.655129
# rank2 -0.243768 -0.344846 0.535481
# rank3 0.468007 -0.250702 0.079670
# rank1 -1.491950 0.690749 -2.023808
按顺序进行多列降序排序
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(4, 3), index=['rank2', 'rank1', 'rank4', 'rank3'], columns=['col3', 'col2', 'col1'])
print(df)
df = df.sort_values(by=['col1', 'col3'], ascending=True, axis=0)
print(df)
# output:
# col3 col2 col1
# rank2 1.035965 1.048124 -0.341586
# rank1 2.391899 -1.575462 0.616940
# rank4 0.968523 -0.932288 -0.553498
# rank3 0.585521 1.907344 -0.264500
# col3 col2 col1
# rank4 0.968523 -0.932288 -0.553498
# rank2 1.035965 1.048124 -0.341586
# rank3 0.585521 1.907344 -0.264500
# rank1 2.391899 -1.575462 0.616940
2、分组
Pandas可以使用groupby函数对DataFrame进行拆分，得到分组对象。
df.groupby(self, by=None, axis=0, level=None, as_index=True, sort=True,
group_keys=True, squeeze=False, observed=False, **kwargs)
by:分组方式，可以是字典、函数、标签、标签列表
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [['Alex', 24, 80], ['Bob', 25, 90], ['Bauer', 25, 90], ['Jack', 26, 80]]
df = pd.DataFrame(data, index=['a', 'b', 'c', 'd'], columns=['Name', 'Age', 'A'])
print(df)
group_obj1 = df.groupby('Name')
print(group_obj1.groups)
print('===================================')
# 单层分组迭代
for key, data in group_obj1:
print(key)
print(data)
group_obj2 = df.groupby(['Name', 'A'])
# 分组信息查看
print(group_obj2.groups)
print('===================================')
# 多层分组迭代
for key, data in group_obj2:
print(key)
print(data)
# output:
# Name Age A
# a Alex 24 80
# b Bob 25 90
# c Bauer 25 90
# d Jack 26 80
# {'Alex': Index(['a'], dtype='object'), 'Bauer': Index(['c'], dtype='object'), 'Bob': Index(['b'], dtype='object'), 'Jack': Index(['d'], dtype='object')}
# ===================================
# Alex
# Name Age A
# a Alex 24 80
# Bauer
# Name Age A
# c Bauer 25 90
# Bob
# Name Age A
# b Bob 25 90
# Jack
# Name Age A
# d Jack 26 80
# {('Alex', 80): Index(['a'], dtype='object'), ('Bauer', 90): Index(['c'], dtype='object'), ('Bob', 90): Index(['b'], dtype='object'), ('Jack', 80): Index(['d'], dtype='object')}
# ===================================
# ('Alex', 80)
# Name Age A
# a Alex 24 80
# ('Bauer', 90)
# Name Age A
# c Bauer 25 90
# ('Bob', 90)
# Name Age A
# b Bob 25 90
# ('Jack', 80)
# Name Age A
# d Jack 26 80
filter()函数可以用于过滤数据。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [['Alex', 24, 80], ['Bob', 25, 92], ['Bauer', 25, 90], ['Jack', 26, 80]]
df = pd.DataFrame(data, index=['a', 'b', 'c', 'd'], columns=['Name', 'Age', 'A'])
print(df)
group_obj1 = df.groupby('Age')
print(group_obj1.groups)
# 过滤年龄相同的人
group = group_obj1.filter(lambda x: len(x) > 1)
print(group)
# output:
# Name Age A
# a Alex 24 80
# b Bob 25 92
# c Bauer 25 90
# d Jack 26 80
# {24: Index(['a'], dtype='object'), 25: Index(['b', 'c'], dtype='object'), 26: Index(['d'], dtype='object')}
# Name Age A
# b Bob 25 92
# c Bauer 25 90
3、合并
pd.merge(left, right, how='inner', on=None, left_on=None, right_on=None, left_index=False, right_index=False, sort=True)
合并两个DataFrame对象。
left ，左DataFrame对象。
right，右DataFrame对象。
on，列(名称)连接，必须在左DataFrame和右DataFrame对象中存在(找到)。
left_on，左侧DataFrame中的列用作键，可以是列名或长度等于DataFrame长度的数组。
right_on，来自右DataFrame的列作为键，可以是列名或长度等于DataFrame长度的数组。
left_index，如果为True，则使用左侧DataFrame中的索引(行标签)作为其连接键。 在具有MultiIndex(分层)的DataFrame的情况下，级别的数量必须与来自右DataFrame的连接键的数量相匹配。
right_index ，与右DataFrame的left_index具有相同的用法。
how，可选值为left, right, outer，inner，默认为inner。
sort，按照字典顺序通过连接键对结果DataFrame进行排序。默认为True，设置为False时，可以大大提高性能。
在一个键上合并两个DataFrame的示例如下：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [['Alex', 24, 80], ['Bob', 25, 90], ['Bauer', 25, 90]]
left = pd.DataFrame(data1, columns=['Name', 'Age', 'A'])
data2 = [['Alex', 87, 78], ['Bob', 67, 87], ['Bauer', 98, 78]]
right = pd.DataFrame(data2, columns=['Name', 'B', 'C'])
print(left)
print('==================================')
print(right)
print('==================================')
df = pd.merge(left, right, on='Name')
print(df)
# output:
# Name Age A
# 0 Alex 24 80
# 1 Bob 25 90
# 2 Bauer 25 90
# ==================================
# Name B C
# 0 Alex 87 78
# 1 Bob 67 87
# 2 Bauer 98 78
# ==================================
# Name Age A B C
# 0 Alex 24 80 87 78
# 1 Bob 25 90 67 87
# 2 Bauer 25 90 98 78
合并多个键上的两个DataFrame的示例如下：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
left = pd.DataFrame(data1, columns=['ID', 'Name', 'Age', 'A'])
data2 = [[1, 'Alex', 87, 78], [4, 'Bob', 67, 87], [3, 'Bauer', 98, 78]]
right = pd.DataFrame(data2, columns=['ID', 'Name', 'B', 'C'])
print(left)
print('==================================')
print(right)
print('==================================')
df = pd.merge(left, right, on=['ID', 'Name'])
print(df)
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ==================================
# ID Name B C
# 0 1 Alex 87 78
# 1 4 Bob 67 87
# 2 3 Bauer 98 78
# ==================================
# ID Name Age A B C
# 0 1 Alex 24 80 87 78
# 1 3 Bauer 25 90 98 78
使用“how”参数进行合并，如何合并参数指定如何确定哪些键将被包含在结果表中。如果组合键没有出现在左侧或右侧表中，则连接表中的值将为NA。
left：LEFT OUTER JOIN，使用左侧对象的键。
right：RIGHT OUTER JOIN，使用右侧对象的键。
outer：FULL OUTER JOIN，使用键的联合。
inner：INNER JOIN，使用键的交集。
Left Join示例：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
left = pd.DataFrame(data1, columns=['ID', 'Name', 'Age', 'A'])
data2 = [[1, 'Alex', 87, 78], [4, 'Bob', 67, 87], [3, 'Bauer', 98, 78]]
right = pd.DataFrame(data2, columns=['ID', 'Name', 'B', 'C'])
print(left)
print('==================================')
print(right)
print('==================================')
df = pd.merge(left, right, on='ID', how='left')
print(df)
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ==================================
# ID Name B C
# 0 1 Alex 87 78
# 1 4 Bob 67 87
# 2 3 Bauer 98 78
# ==================================
# ID Name_x Age A Name_y B C
# 0 1 Alex 24 80 Alex 87.0 78.0
# 1 2 Bob 25 90 NaN NaN NaN
# 2 3 Bauer 25 90 Bauer 98.0 78.0
Right Join示例：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
left = pd.DataFrame(data1, columns=['ID', 'Name', 'Age', 'A'])
data2 = [[1, 'Alex', 87, 78], [4, 'Bob', 67, 87], [3, 'Bauer', 98, 78]]
right = pd.DataFrame(data2, columns=['ID', 'Name', 'B', 'C'])
print(left)
print('==================================')
print(right)
print('==================================')
df = pd.merge(left, right, on='ID', how='right')
print(df)
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ==================================
# ID Name B C
# 0 1 Alex 87 78
# 1 4 Bob 67 87
# 2 3 Bauer 98 78
# ==================================
# ID Name_x Age A Name_y B C
# 0 1 Alex 24.0 80.0 Alex 87 78
# 1 3 Bauer 25.0 90.0 Bauer 98 78
# 2 4 NaN NaN NaN Bob 67 87
Outer Join示例：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
left = pd.DataFrame(data1, columns=['ID', 'Name', 'Age', 'A'])
data2 = [[1, 'Alex', 87, 78], [4, 'Bob', 67, 87], [3, 'Bauer', 98, 78]]
right = pd.DataFrame(data2, columns=['ID', 'Name', 'B', 'C'])
print(left)
print('==================================')
print(right)
print('==================================')
df = pd.merge(left, right, on='ID', how='outer')
print(df)
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ==================================
# ID Name B C
# 0 1 Alex 87 78
# 1 4 Bob 67 87
# 2 3 Bauer 98 78
# ==================================
# ID Name_x Age A Name_y B C
# 0 1 Alex 24.0 80.0 Alex 87.0 78.0
# 1 2 Bob 25.0 90.0 NaN NaN NaN
# 2 3 Bauer 25.0 90.0 Bauer 98.0 78.0
# 3 4 NaN NaN NaN Bob 67.0 87.0
Inner Join示例：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
left = pd.DataFrame(data1, columns=['ID', 'Name', 'Age', 'A'])
data2 = [[1, 'Alex', 87, 78], [4, 'Bob', 67, 87], [3, 'Bauer', 98, 78]]
right = pd.DataFrame(data2, columns=['ID', 'Name', 'B', 'C'])
print(left)
print('==================================')
print(right)
print('==================================')
df = pd.merge(left, right, on='ID', how='inner')
print(df)
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ==================================
# ID Name B C
# 0 1 Alex 87 78
# 1 4 Bob 67 87
# 2 3 Bauer 98 78
# ==================================
# ID Name_x Age A Name_y B C
# 0 1 Alex 24 80 Alex 87 78
# 1 3 Bauer 25 90 Bauer 98 78
4、级联
concat(objs, axis=0, join='outer', join_axes=None, ignore_index=False,
keys=None, levels=None, names=None, verify_integrity=False,
sort=None, copy=True)
沿某个轴进行级联操作。
objs，Series、DataFrame或Panel对象的序列或字典。
axis，{0，1，...}，默认为0，axis=0表示按index进行级联，axis=1表示按columns进行级联。
join，{'inner', 'outer'}，默认inner，指示如何处理其它轴上的索引。
ignore_index，布尔值，默认为False。如果指定为True，则不使用连接轴上的索引值。结果轴将被标记为：0，...，n-1。
join_axes ，Index对象的列表。用于其它(n-1)轴的特定索引，而不是执行内部/外部集逻辑。
sort:是否进行排序，True会进行排序，False不进行排序。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [['Alex', 24, 80], ['Bob', 25, 90], ['Bauer', 25, 90]]
one = pd.DataFrame(data1, columns=['Name', 'Age', 'A'])
data2 = [['Alex', 87, 78], ['Bob', 67, 87], ['Bauer', 98, 78]]
two = pd.DataFrame(data2, columns=['Name', 'B', 'C'])
print(one)
print('==================================')
print(two)
print('==================================')
df = pd.concat([one, two], axis=1, sort=False)
print(df)
# output:
# Name Age A
# 0 Alex 24 80
# 1 Bob 25 90
# 2 Bauer 25 90
# ==================================
# Name B C
# 0 Alex 87 78
# 1 Bob 67 87
# 2 Bauer 98 78
# ==================================
# Name Age A Name B C
# 0 Alex 24 80 Alex 87 78
# 1 Bob 25 90 Bob 67 87
# 2 Bauer 25 90 Bauer 98 78
当结果的索引是重复的，如果想要生成的对象必须遵循自己的索引，需要将ignore_index设置为True。
Pandas提供了连接DataFrame的append方法，沿axis=0连接。
df.append(self, other, ignore_index=False,
verify_integrity=False, sort=None)
向DataFrame对象中添加新的行，如果添加的列名不在DataFrame对象中，将会被当作新的列进行添加。
other：DataFrame、series、dict、list
ignore_index：默认值为False，如果为True则不使用index标签。
verify_integrity ：默认值为False，如果为True当创建相同的index时会抛出ValueError的异常。
sort：boolean，默认是None。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [['Alex', 24, 80], ['Bob', 25, 90], ['Bauer', 25, 90]]
one = pd.DataFrame(data1, columns=['Name', 'Age', 'A'])
data2 = [['Alex', 87, 78], ['Bob', 67, 87], ['Bauer', 98, 78]]
two = pd.DataFrame(data2, columns=['Name', 'B', 'C'])
print(one)
print('==================================')
print(two)
print('==================================')
df = one.append(two, sort=False)
print(df)
# output:
# Name Age A
# 0 Alex 24 80
# 1 Bob 25 90
# 2 Bauer 25 90
# ==================================
# Name B C
# 0 Alex 87 78
# 1 Bob 67 87
# 2 Bauer 98 78
# ==================================
# Name Age A B C
# 0 Alex 24.0 80.0 NaN NaN
# 1 Bob 25.0 90.0 NaN NaN
# 2 Bauer 25.0 90.0 NaN NaN
# 0 Alex NaN NaN 87.0 78.0
# 1 Bob NaN NaN 67.0 87.0
# 2 Bauer NaN NaN 98.0 78.0
Pandas提供了连接DataFrame的join方法，沿axis=1连接，用于将两个DataFrame中的不同的列索引合并成为一个DataFrame。
df.join(self, other, on=None, how='left', lsuffix='', rsuffix='',
sort=False)
join方法提供SQL的Join操作，默认为为左外连接how=left。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data1 = [['Alex', 24, 80], ['Bob', 25, 90], ['Bauer', 25, 90],['Jack', 26, 80]]
one = pd.DataFrame(data1, index=['a', 'b', 'c', 'd'], columns=['Name', 'Age', 'A'])
data2 = [[87, 78], [67, 87], [98, 78]]
two = pd.DataFrame(data2, index=['a', 'b', 'c'], columns=['B', 'C'])
print(one)
print('==================================')
print(two)
print('==================================')
df = one.join(two)
print(df)
# output:
# Name Age A
# a Alex 24 80
# b Bob 25 90
# c Bauer 25 90
# d Jack 26 80
# ==================================
# B C
# a 87 78
# b 67 87
# c 98 78
# ==================================
# Name Age A B C
# a Alex 24 80 87.0 78.0
# b Bob 25 90 67.0 87.0
# c Bauer 25 90 98.0 78.0
# d Jack 26 80 NaN NaN
5、迭代
迭代DataFrame提供列名。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
print(df)
for col in df:
print(col, end=' ')
# output:
# A B C D
# 2019-01-01 -0.415754 -1.214340 -0.103952 1.232414
# 2019-01-02 -0.367888 0.257199 -1.615029 -0.335322
# 2019-01-03 0.552697 0.202993 -1.000219 -0.530897
# 2019-01-04 0.503410 -1.610091 1.660362 0.649700
# 2019-01-05 0.575416 -1.962578 -1.681379 -0.425239
# 2019-01-06 1.075917 -0.499081 1.886878 -0.073895
# A B C D
df.iteritems()用于迭代(key，value)对，将每个列标签作为key，value为Series对象。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
dates = pd.date_range('20190101', periods=6)
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
print(df)
for key, value in df.iteritems():
print(key, value)
# output:
# A B C D
# 2019-01-01 -0.302021 1.343811 -0.070351 -0.409479
# 2019-01-02 -0.365564 0.743572 -0.475075 1.026054
# 2019-01-03 0.025748 1.395340 -0.987686 0.141003
# 2019-01-04 -0.291348 -1.173600 -2.286905 0.528416
# 2019-01-05 -1.844523 -0.052567 0.575980 0.260001
# 2019-01-06 0.271046 -0.583334 -0.596251 0.772095
# A 2019-01-01 -0.302021
# 2019-01-02 -0.365564
# 2019-01-03 0.025748
# 2019-01-04 -0.291348
# 2019-01-05 -1.844523
# 2019-01-06 0.271046
# Freq: D, Name: A, dtype: float64
# B 2019-01-01 1.343811
# 2019-01-02 0.743572
# 2019-01-03 1.395340
# 2019-01-04 -1.173600
# 2019-01-05 -0.052567
# 2019-01-06 -0.583334
# Freq: D, Name: B, dtype: float64
# C 2019-01-01 -0.070351
# 2019-01-02 -0.475075
# 2019-01-03 -0.987686
# 2019-01-04 -2.286905
# 2019-01-05 0.575980
# 2019-01-06 -0.596251
# Freq: D, Name: C, dtype: float64
# D 2019-01-01 -0.409479
# 2019-01-02 1.026054
# 2019-01-03 0.141003
# 2019-01-04 0.528416
# 2019-01-05 0.260001
# 2019-01-06 0.772095
# Freq: D, Name: D, dtype: float64
df.iterrows()用于返回迭代器，产生每个index以及包含每行数据的Series。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(6, 4), columns=list('ABCD'))
print(df)
for index, value in df.iterrows():
print(index, value)
# output:
# A B C D
# 0 -1.097851 0.785749 -1.727198 -1.120925
# 1 -1.420429 0.094384 -1.566202 0.237084
# 2 -0.761957 0.552395 0.680884 -0.290955
# 3 0.357713 -0.323331 1.438013 -1.334616
# 4 0.015467 -2.431556 -0.717285 -0.094409
# 5 -1.198224 -1.370170 0.201725 0.258093
# 0 A -1.097851
# B 0.785749
# C -1.727198
# D -1.120925
# Name: 0, dtype: float64
# 1 A -1.420429
# B 0.094384
# C -1.566202
# D 0.237084
# Name: 1, dtype: float64
# 2 A -0.761957
# B 0.552395
# C 0.680884
# D -0.290955
# Name: 2, dtype: float64
# 3 A 0.357713
# B -0.323331
# C 1.438013
# D -1.334616
# Name: 3, dtype: float64
# 4 A 0.015467
# B -2.431556
# C -0.717285
# D -0.094409
# Name: 4, dtype: float64
# 5 A -1.198224
# B -1.370170
# C 0.201725
# D 0.258093
# Name: 5, dtype: float64
df.itertuples()方法将为DataFrame中的每一行返回一个产生一个命名元组的迭代器。元组的第一个元素是行的index，而剩余的值是行值。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(6, 4), columns=list('ABCD'))
print(df)
for row in df.itertuples():
print(row)
# output:
# A B C D
# 0 0.681324 1.047734 -1.909570 -0.845900
# 1 -0.879077 -0.897085 -0.795461 -0.634519
# 2 0.484502 -0.061608 0.605827 -0.321721
# 3 -0.051974 1.533112 -1.011544 -0.922280
# 4 -0.634157 -0.173692 1.228584 -1.229581
# 5 0.236769 -0.933609 0.111948 1.048215
# Pandas(Index=0, A=0.6813238552921729, B=1.0477343302788706, C=-1.909570436815022, D=-0.8459001766064564)
# Pandas(Index=1, A=-0.8790771200969485, B=-0.8970849190216943, C=-0.7954606477323869, D=-0.6345188867416923)
# Pandas(Index=2, A=0.48450157948338324, B=-0.061608014575315506, C=0.6058267522125123, D=-0.32172144100965605)
# Pandas(Index=3, A=-0.05197447447575398, B=1.5331115391025778, C=-1.0115444345763995, D=-0.9222798204619236)
# Pandas(Index=4, A=-0.6341570074338677, B=-0.173692444412635, C=1.2285839004083785, D=-1.2295807166909738)
# Pandas(Index=5, A=0.23676890089548117, B=-0.9336090868233837, C=0.11194794444517034, D=1.0482154173833818)
迭代用于读取，迭代器返回原始对象(视图)的副本，因此迭代时更改将不会反映在原始对象上。
6、SQL化操作
在SQL中，SELECT使用逗号分隔的列列表(或选择所有列)来完成。
SELECT ID, Name FROM tablename LIMIT 5;
在Pandas中，列选择通过传递列名到DataFrame。
df[['ID', 'Name']].head(5)
SELECT操作示例：
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
df = pd.DataFrame(data, columns=['ID', 'Name', 'Age', 'A'])
print(df)
print(df[['ID', 'Name']].head(5))
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ID Name
# 0 1 Alex
# 1 2 Bob
# 2 3 Bauer
在SQL中，使用WHERE进行条件过滤。
SELECT * FROM tablename WHERE Name = 'Bauer' LIMIT 5;
在Pandas中，通常使用布尔索引进行过滤。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
data = [[1, 'Alex', 24, 80], [2, 'Bob', 25, 90], [3, 'Bauer', 25, 90]]
df = pd.DataFrame(data, columns=['ID', 'Name', 'Age', 'A'])
print(df)
print('===========================')
print(df[df['Name'] == 'Bauer'].head(5))
# output:
# ID Name Age A
# 0 1 Alex 24 80
# 1 2 Bob 25 90
# 2 3 Bauer 25 90
# ===========================
# ID Name Age A
# 2 3 Bauer 25 90
四、数据分析
1、描述性统计
（1）sum
返回所请求轴的值的总和。 默认情况下，轴为索引(axis=0)。
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.sum())
print(df.sum(1))
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Name AlexBobBauer
# Age 75
# Score 257
# dtype: object
# 0 105
# 1 116
# 2 111
# dtype: int64
（2）mean
返回平均值。
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.mean())
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Age 25.000000
# Score 85.666667
# dtype: float64
（3）std
返回数字列的Bressel标准偏差。
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.std())
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Age 1.000000
# Score 5.131601
# dtype: float64
（4）median
求所有值的中位数。
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.median())
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Age 25.0
# Score 87.0
# dtype: float64
（5）min
求所有值中的最小值。
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.min())
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Name Alex
# Age 24
# Score 80
# dtype: object
（6）max
求所有值中的最大值。
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.max())
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Name Bob
# Age 26
# Score 90
# dtype: object
（7）describe
汇总有关DataFrame列的统计信息的摘要。
def describe(self, percentiles=None, include=None, exclude=None)
include用于传递关于什么列需要考虑用于总结的必要信息的参数。获取值列表，默认情况下是number 。
object - 汇总字符串列
number - 汇总数字列
all - 将所有列汇总在一起(不应将其作为列表值传递)
import pandas as pd
if __name__ == "__main__":
data = [['Alex', 25, 80], ['Bob', 26, 90], ['Bauer', 24, 87]]
df = pd.DataFrame(data, columns=['Name', 'Age', 'Score'])
print(df)
print(df.describe(include="all"))
# output:
# Name Age Score
# 0 Alex 25 80
# 1 Bob 26 90
# 2 Bauer 24 87
# Name Age Score
# count 3 3.0 3.000000
# unique 3 NaN NaN
# top Alex NaN NaN
# freq 1 NaN NaN
# mean NaN 25.0 85.666667
# std NaN 1.0 5.131601
# min NaN 24.0 80.000000
# 25% NaN 24.5 83.500000
# 50% NaN 25.0 87.000000
# 75% NaN 25.5 88.500000
# max NaN 26.0 90.000000
abs：求所有值的绝对值
prod：求所有值的乘积
cumsum：累计总和
cumprod：累计乘积
2、百分比变化
Series，DatFrames和Panel都有pct_change()函数，用于将每个元素与其前一个元素进行比较，并计算变化百分比。默认情况下，pct_change()对列进行操作; 如果想应用到行上，那么可使用axis = 1参数。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(4, 3), index=['rank2', 'rank1', 'rank4', 'rank3'], columns=['col3', 'col2', 'col1'])
print(df)
print(df.pct_change())
# output:
# col3 col2 col1
# rank2 0.988739 2.062798 1.400892
# rank1 0.394663 -0.988307 1.583098
# rank4 -0.768109 -0.163727 -1.801323
# rank3 0.999816 -1.224068 1.470020
# col3 col2 col1
# rank2 NaN NaN NaN
# rank1 -0.600842 -1.479110 0.130064
# rank4 -2.946241 -0.834336 -2.137846
# rank3 -2.301659 6.476294 -1.816078
3、协方差
协方差适用于Series数据，Series对象有一个方法cov用来计算Series对象之间的协方差，NA将被自动排除。当应用于DataFrame对象时，协方差方法计算所有列之间的协方差(cov)值。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(3, 5), columns=['a', 'b', 'c', 'd', 'e'])
print(df)
print(df['a'].cov(df['b']))
print(df.cov())
# output:
# a b c d e
# 0 1.168443 -0.343905 2.254448 0.269765 -0.928009
# 1 0.542551 -1.303205 -1.767313 -0.349884 -0.352578
# 2 -2.028410 -1.176339 0.156047 1.426468 -1.338805
# 0.48923631972868176
# a b c d e
# a 2.870241 0.489236 0.713430 -1.312818 0.581441
# b 0.489236 0.271550 0.974811 -0.023849 -0.055862
# c 0.713430 0.974811 4.046193 0.580236 -0.558184
# d -1.312818 -0.023849 0.580236 0.812892 -0.430603
# e 0.581441 -0.055862 -0.558184 -0.430603 0.245420
4、相关性
相关性显示了任何两个数值(Series)之间的线性关系。有多种计算相关性的方法，如pearson(默认)，spearman和kendall。如果DataFrame中存在任何非数字列，则会自动排除。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
df = pd.DataFrame(np.random.randn(3, 5), columns=['a', 'b', 'c', 'd', 'e'])
print(df)
print(df['a'].corr(df['b']))
print(df.corr())
# output:
# a b c d e
# 0 -2.110756 0.693665 0.405701 -0.628349 -1.062029
# 1 -1.331364 1.283434 1.619166 -0.025866 1.742287
# 2 -1.159944 0.435840 -0.251710 -0.347102 -0.026825
# 0.052396578025987336
# a b c d e
# a 1.000000 0.052397 -0.000006 0.743940 0.664845
# b 0.052397 1.000000 0.998626 0.706309 0.780790
# c -0.000006 0.998626 1.000000 0.668242 0.746977
# d 0.743940 0.706309 0.668242 1.000000 0.993772
# e 0.664845 0.780790 0.746977 0.993772 1.000000
5、数据排名
数据排名为元素数组中的每个元素生成排名。在关系的情况下，分配平均等级。
# -*- coding=utf-8 -*-
import pandas as pd
import numpy as np
if __name__ == "__main__":
s = pd.Series(np.random.randn(5), index=list('abcde'))
print(s)
s['a'] = s['c']
print(s.rank())
# output:
# a 1.597684
# a 1.597684
# b 1.107413
# c -0.298296
# d -0.281076
# e -0.667954
# dtype: float64
# a 2.5
# b 5.0
# c 2.5
# d 4.0
# e 1.0
# dtype: float64
rank使用一个默认为True的升序参数; False时，数据被反向排序，较大的值被分配较小的排序。
网站名称：Python3快速入门（十五）——Pandas数据处理
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