10 Minutes to pandas----十分钟搞定Pandas

本文是对官方网站上《10 Minutes to pandas》的一个简单的翻译，原文地址是：http://pandas.pydata.org/pandas-docs/stable/10min.html

这篇文章是对pandas的一个简单的介绍，由于出身非英语专业，错误在所难免，还望指正。

中肯建议：英语水平只要是还说的过去的，建议看官方文档，翻译的毕竟和原文有出入！！！

习惯上，我们会按下面格式引入所需要的包：

In [1]: import pandas as pdIn [2]: import numpy as npIn [3]: import matplotlib.pyplot as plt

一、创建对象

1、可以通过传递一个list对象来创建一个Series，pandas会默认创建整型索引：

In [4]: s = pd.Series([1,3,5,np.nan,6,8])In [5]: sOut[5]: 0    1.01    3.02    5.03    NaN4    6.05    8.0dtype: float64

2、通过传递一个numpy array，时间索引以及列标签来创建一个DataFrame：

In [6]: dates = pd.date_range('20130101', periods=6)In [7]: datesOut[7]: DatetimeIndex(['2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04',               '2013-01-05', '2013-01-06'],              dtype='datetime64[ns]', freq='D')In [8]: df = pd.DataFrame(np.random.randn(6,4), index=dates, columns=list('ABCD'))In [9]: dfOut[9]:                    A         B         C         D2013-01-01  0.469112 -0.282863 -1.509059 -1.1356322013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-03 -0.861849 -2.104569 -0.494929  1.0718042013-01-04  0.721555 -0.706771 -1.039575  0.2718602013-01-05 -0.424972  0.567020  0.276232 -1.0874012013-01-06 -0.673690  0.113648 -1.478427  0.524988

3、通过传递一个能够被转换成类似序列结构的字典对象来创建一个DataFrame：

In [10]: df2 = pd.DataFrame({ 'A' : 1.,   ....:                      'B' : pd.Timestamp('20130102'),   ....:                      'C' : pd.Series(1,index=list(range(4)),dtype='float32'),   ....:                      'D' : np.array([3] * 4,dtype='int32'),   ....:                      'E' : pd.Categorical(["test","train","test","train"]),   ....:                      'F' : 'foo' })   ....: In [11]: df2Out[11]:      A          B    C  D      E    F0  1.0 2013-01-02  1.0  3   test  foo1  1.0 2013-01-02  1.0  3  train  foo2  1.0 2013-01-02  1.0  3   test  foo3  1.0 2013-01-02  1.0  3  train  foo

4、查看不同列的数据类型：

In [12]: df2.dtypesOut[12]: A           float64B    datetime64[ns]C           float32D             int32E          categoryF            objectdtype: object

5、如果你使用的是IPython，使用Tab自动补全功能会自动识别所有的属性以及自定义的列，下图中是所有能够被自动识别的属性的一个子集：

In [13]: df2.<TAB>df2.A                  df2.boxplotdf2.abs                df2.Cdf2.add                df2.clipdf2.add_prefix         df2.clip_lowerdf2.add_suffix         df2.clip_upperdf2.align              df2.columnsdf2.all                df2.combinedf2.any                df2.combineAdddf2.append             df2.combine_firstdf2.apply              df2.combineMultdf2.applymap           df2.compounddf2.as_blocks          df2.consolidatedf2.asfreq             df2.convert_objectsdf2.as_matrix          df2.copydf2.astype             df2.corrdf2.at                 df2.corrwithdf2.at_time            df2.countdf2.axes               df2.covdf2.B                  df2.cummaxdf2.between_time       df2.cummindf2.bfill              df2.cumproddf2.blocks             df2.cumsumdf2.bool               df2.D

正如你所看到的,列A,B,C,D的标签自动生成，E的也是。而其余的属性已经做了简要的截断。

二、查看数据

1、  查看frame中头部和尾部的行：

In [14]: df.head()Out[14]:                    A         B         C         D2013-01-01  0.469112 -0.282863 -1.509059 -1.1356322013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-03 -0.861849 -2.104569 -0.494929  1.0718042013-01-04  0.721555 -0.706771 -1.039575  0.2718602013-01-05 -0.424972  0.567020  0.276232 -1.087401In [15]: df.tail(3)Out[15]:                    A         B         C         D2013-01-04  0.721555 -0.706771 -1.039575  0.2718602013-01-05 -0.424972  0.567020  0.276232 -1.0874012013-01-06 -0.673690  0.113648 -1.478427  0.524988

2、  显示索引、列和底层的numpy数据：

In [16]: df.indexOut[16]: DatetimeIndex(['2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04',               '2013-01-05', '2013-01-06'],              dtype='datetime64[ns]', freq='D')In [17]: df.columnsOut[17]: Index([u'A', u'B', u'C', u'D'], dtype='object')In [18]: df.valuesOut[18]: array([[ 0.4691, -0.2829, -1.5091, -1.1356],       [ 1.2121, -0.1732,  0.1192, -1.0442],       [-0.8618, -2.1046, -0.4949,  1.0718],       [ 0.7216, -0.7068, -1.0396,  0.2719],       [-0.425 ,  0.567 ,  0.2762, -1.0874],       [-0.6737,  0.1136, -1.4784,  0.525 ]])

3、  describe()函数对于数据的快速统计汇总：

In [19]: df.describe()Out[19]:               A         B         C         Dcount  6.000000  6.000000  6.000000  6.000000mean   0.073711 -0.431125 -0.687758 -0.233103std    0.843157  0.922818  0.779887  0.973118min   -0.861849 -2.104569 -1.509059 -1.13563225%   -0.611510 -0.600794 -1.368714 -1.07661050%    0.022070 -0.228039 -0.767252 -0.38618875%    0.658444  0.041933 -0.034326  0.461706max    1.212112  0.567020  0.276232  1.071804

4、  对数据的转置：

In [20]: df.TOut[20]:    2013-01-01  2013-01-02  2013-01-03  2013-01-04  2013-01-05  2013-01-06A    0.469112    1.212112   -0.861849    0.721555   -0.424972   -0.673690B   -0.282863   -0.173215   -2.104569   -0.706771    0.567020    0.113648C   -1.509059    0.119209   -0.494929   -1.039575    0.276232   -1.478427D   -1.135632   -1.044236    1.071804    0.271860   -1.087401    0.524988

5、  按轴进行排序

In [21]: df.sort_index(axis=1, ascending=False)Out[21]:                    D         C         B         A2013-01-01 -1.135632 -1.509059 -0.282863  0.4691122013-01-02 -1.044236  0.119209 -0.173215  1.2121122013-01-03  1.071804 -0.494929 -2.104569 -0.8618492013-01-04  0.271860 -1.039575 -0.706771  0.7215552013-01-05 -1.087401  0.276232  0.567020 -0.4249722013-01-06  0.524988 -1.478427  0.113648 -0.673690

6、  按值进行排序

In [22]: df.sort_values(by='B')Out[22]:                    A         B         C         D2013-01-03 -0.861849 -2.104569 -0.494929  1.0718042013-01-04  0.721555 -0.706771 -1.039575  0.2718602013-01-01  0.469112 -0.282863 -1.509059 -1.1356322013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-06 -0.673690  0.113648 -1.478427  0.5249882013-01-05 -0.424972  0.567020  0.276232 -1.087401

三、选择

虽然标准的Python/Numpy的选择和设置表达式都能够直接派上用场，但是作为工程使用的代码，我们推荐使用经过优化的pandas数据访问方式： .at, .iat, .loc, .iloc 和 .ix

l  获取

1、 选择一个单独的列，这将会返回一个Series，等同于df.A：

In [23]: df['A']Out[23]: 2013-01-01    0.4691122013-01-02    1.2121122013-01-03   -0.8618492013-01-04    0.7215552013-01-05   -0.4249722013-01-06   -0.673690Freq: D, Name: A, dtype: float64

2、 通过[]进行选择，这将会对行进行切片

In [24]: df[0:3]Out[24]:                    A         B         C         D2013-01-01  0.469112 -0.282863 -1.509059 -1.1356322013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-03 -0.861849 -2.104569 -0.494929  1.071804In [25]: df['20130102':'20130104']Out[25]:                    A         B         C         D2013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-03 -0.861849 -2.104569 -0.494929  1.0718042013-01-04  0.721555 -0.706771 -1.039575  0.271860

l  通过标签选择

1、 使用标签来获取一个交叉的区域

In [26]: df.loc[dates[0]]Out[26]: A    0.469112B   -0.282863C   -1.509059D   -1.135632Name: 2013-01-01 00:00:00, dtype: float64

2、 通过标签来在多个轴上进行选择

In [27]: df.loc[:,['A','B']]Out[27]:                    A         B2013-01-01  0.469112 -0.2828632013-01-02  1.212112 -0.1732152013-01-03 -0.861849 -2.1045692013-01-04  0.721555 -0.7067712013-01-05 -0.424972  0.5670202013-01-06 -0.673690  0.113648

3、 标签切片

In [28]: df.loc['20130102':'20130104',['A','B']]Out[28]:                    A         B2013-01-02  1.212112 -0.1732152013-01-03 -0.861849 -2.1045692013-01-04  0.721555 -0.706771

4、 对于返回的对象进行维度缩减

In [29]: df.loc['20130102',['A','B']]Out[29]: A    1.212112B   -0.173215Name: 2013-01-02 00:00:00, dtype: float64

5、 获取一个标量

In [30]: df.loc[dates[0],'A']Out[30]: 0.46911229990718628

6、 快速访问一个标量（与上一个方法等价）

In [31]: df.at[dates[0],'A']Out[31]: 0.46911229990718628

l  通过位置选择

1、 通过传递数值进行位置选择（选择的是行）

In [32]: df.iloc[3]Out[32]: A    0.721555B   -0.706771C   -1.039575D    0.271860Name: 2013-01-04 00:00:00, dtype: float64

2、 通过数值进行切片，与numpy/python中的情况类似

In [33]: df.iloc[3:5,0:2]Out[33]:                    A         B2013-01-04  0.721555 -0.7067712013-01-05 -0.424972  0.567020

3、 通过指定一个位置的列表，与numpy/python中的情况类似

In [34]: df.iloc[[1,2,4],[0,2]]Out[34]:                    A         C2013-01-02  1.212112  0.1192092013-01-03 -0.861849 -0.4949292013-01-05 -0.424972  0.276232

4、 对行进行切片

In [35]: df.iloc[1:3,:]Out[35]:                    A         B         C         D2013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-03 -0.861849 -2.104569 -0.494929  1.071804

5、 对列进行切片

In [36]: df.iloc[:,1:3]Out[36]:                    B         C2013-01-01 -0.282863 -1.5090592013-01-02 -0.173215  0.1192092013-01-03 -2.104569 -0.4949292013-01-04 -0.706771 -1.0395752013-01-05  0.567020  0.2762322013-01-06  0.113648 -1.478427

6、 获取特定的值

In [37]: df.iloc[1,1]Out[37]: -0.17321464905330858

获得快速访问的一个标量（等同于以前的做法）

In [38]: df.iat[1,1]Out[38]: -0.17321464905330858

l  布尔索引

1、 使用一个单独列的值来选择数据：

In [39]: df[df.A > 0]Out[39]:                    A         B         C         D2013-01-01  0.469112 -0.282863 -1.509059 -1.1356322013-01-02  1.212112 -0.173215  0.119209 -1.0442362013-01-04  0.721555 -0.706771 -1.039575  0.271860

2、 使用where操作来选择数据：

In [40]: df[df > 0]Out[40]:                    A         B         C         D2013-01-01  0.469112       NaN       NaN       NaN2013-01-02  1.212112       NaN  0.119209       NaN2013-01-03       NaN       NaN       NaN  1.0718042013-01-04  0.721555       NaN       NaN  0.2718602013-01-05       NaN  0.567020  0.276232       NaN2013-01-06       NaN  0.113648       NaN  0.524988

3、 使用isin()方法来过滤：

In [41]: df2 = df.copy()In [42]: df2['E'] = ['one', 'one','two','three','four','three']In [43]: df2Out[43]:                    A         B         C         D      E2013-01-01  0.469112 -0.282863 -1.509059 -1.135632    one2013-01-02  1.212112 -0.173215  0.119209 -1.044236    one2013-01-03 -0.861849 -2.104569 -0.494929  1.071804    two2013-01-04  0.721555 -0.706771 -1.039575  0.271860  three2013-01-05 -0.424972  0.567020  0.276232 -1.087401   four2013-01-06 -0.673690  0.113648 -1.478427  0.524988  threeIn [44]: df2[df2['E'].isin(['two','four'])]Out[44]:                    A         B         C         D     E2013-01-03 -0.861849 -2.104569 -0.494929  1.071804   two2013-01-05 -0.424972  0.567020  0.276232 -1.087401  four

l  设置

1、 设置一个新的列：

In [45]: s1 = pd.Series([1,2,3,4,5,6], index=pd.date_range('20130102', periods=6))In [46]: s1Out[46]: 2013-01-02    12013-01-03    22013-01-04    32013-01-05    42013-01-06    52013-01-07    6Freq: D, dtype: int64In [47]: df['F'] = s1

2、 通过标签设置新的值：

In [48]: df.at[dates[0],'A'] = 0

3、 通过位置设置新的值：

In [49]: df.iat[0,1] = 0

4、 通过一个numpy数组设置一组新值：

In [50]: df.loc[:,'D'] = np.array([5] * len(df))

上述操作结果如下：

In [51]: dfOut[51]:                    A         B         C  D    F2013-01-01  0.000000  0.000000 -1.509059  5  NaN2013-01-02  1.212112 -0.173215  0.119209  5  1.02013-01-03 -0.861849 -2.104569 -0.494929  5  2.02013-01-04  0.721555 -0.706771 -1.039575  5  3.02013-01-05 -0.424972  0.567020  0.276232  5  4.02013-01-06 -0.673690  0.113648 -1.478427  5  5.0

5、 通过where操作来设置新的值：

In [52]: df2 = df.copy()In [53]: df2[df2 > 0] = -df2In [54]: df2Out[54]:                    A         B         C  D    F2013-01-01  0.000000  0.000000 -1.509059 -5  NaN2013-01-02 -1.212112 -0.173215 -0.119209 -5 -1.02013-01-03 -0.861849 -2.104569 -0.494929 -5 -2.02013-01-04 -0.721555 -0.706771 -1.039575 -5 -3.02013-01-05 -0.424972 -0.567020 -0.276232 -5 -4.02013-01-06 -0.673690 -0.113648 -1.478427 -5 -5.0

四、缺失值处理

在pandas中，使用np.nan来代替缺失值，这些值将默认不会包含在计算中。

1、  reindex()方法可以对指定轴上的索引进行改变/增加/删除操作，这将返回原始数据的一个拷贝：、

In [55]: df1 = df.reindex(index=dates[0:4], columns=list(df.columns) + ['E'])In [56]: df1.loc[dates[0]:dates[1],'E'] = 1In [57]: df1Out[57]:                    A         B         C  D    F    E2013-01-01  0.000000  0.000000 -1.509059  5  NaN  1.02013-01-02  1.212112 -0.173215  0.119209  5  1.0  1.02013-01-03 -0.861849 -2.104569 -0.494929  5  2.0  NaN2013-01-04  0.721555 -0.706771 -1.039575  5  3.0  NaN

2、  去掉包含缺失值的行：

In [58]: df1.dropna(how='any')Out[58]:                    A         B         C  D    F    E2013-01-02  1.212112 -0.173215  0.119209  5  1.0  1.0

3、  对缺失值进行填充：

In [59]: df1.fillna(value=5)Out[59]:                    A         B         C  D    F    E2013-01-01  0.000000  0.000000 -1.509059  5  5.0  1.02013-01-02  1.212112 -0.173215  0.119209  5  1.0  1.02013-01-03 -0.861849 -2.104569 -0.494929  5  2.0  5.02013-01-04  0.721555 -0.706771 -1.039575  5  3.0  5.0

4、  对数据进行布尔填充：

In [60]: pd.isnull(df1)Out[60]:                 A      B      C      D      F      E2013-01-01  False  False  False  False   True  False2013-01-02  False  False  False  False  False  False2013-01-03  False  False  False  False  False   True2013-01-04  False  False  False  False  False   True

五、相关操作

l  统计（相关操作通常情况下不包括缺失值）

1、  执行描述性统计：

In [61]: df.mean()Out[61]: A   -0.004474B   -0.383981C   -0.687758D    5.000000F    3.000000dtype: float64

2、  在其他轴上进行相同的操作：

In [62]: df.mean(1)Out[62]: 2013-01-01    0.8727352013-01-02    1.4316212013-01-03    0.7077312013-01-04    1.3950422013-01-05    1.8836562013-01-06    1.592306Freq: D, dtype: float64

3、  对于拥有不同维度，需要对齐的对象进行操作。Pandas会自动的沿着指定的维度进行广播：

In [63]: s = pd.Series([1,3,5,np.nan,6,8], index=dates).shift(2)In [64]: sOut[64]: 2013-01-01    NaN2013-01-02    NaN2013-01-03    1.02013-01-04    3.02013-01-05    5.02013-01-06    NaNFreq: D, dtype: float64In [65]: df.sub(s, axis='index')Out[65]:                    A         B         C    D    F2013-01-01       NaN       NaN       NaN  NaN  NaN2013-01-02       NaN       NaN       NaN  NaN  NaN2013-01-03 -1.861849 -3.104569 -1.494929  4.0  1.02013-01-04 -2.278445 -3.706771 -4.039575  2.0  0.02013-01-05 -5.424972 -4.432980 -4.723768  0.0 -1.02013-01-06       NaN       NaN       NaN  NaN  NaN

l  Apply

1、  对数据应用函数：

In [66]: df.apply(np.cumsum)Out[66]:                    A         B         C   D     F2013-01-01  0.000000  0.000000 -1.509059   5   NaN2013-01-02  1.212112 -0.173215 -1.389850  10   1.02013-01-03  0.350263 -2.277784 -1.884779  15   3.02013-01-04  1.071818 -2.984555 -2.924354  20   6.02013-01-05  0.646846 -2.417535 -2.648122  25  10.02013-01-06 -0.026844 -2.303886 -4.126549  30  15.0In [67]: df.apply(lambda x: x.max() - x.min())Out[67]: A    2.073961B    2.671590C    1.785291D    0.000000F    4.000000dtype: float64

l  直方图

1、直方图

In [68]: s = pd.Series(np.random.randint(0, 7, size=10))In [69]: sOut[69]: 0    41    22    13    24    65    46    47    68    49    4dtype: int64In [70]: s.value_counts()Out[70]: 4    56    22    21    1dtype: int64

l  字符串方法

Series对象在其str属性中配备了一组字符串处理方法，可以很容易的应用到数组中的每个元素，如下段代码所示。

In [71]: s = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat'])In [72]: s.str.lower()Out[72]: 0       a1       b2       c3    aaba4    baca5     NaN6    caba7     dog8     catdtype: object

六、合并

Pandas提供了大量的方法能够轻松的对Series，DataFrame和Panel对象进行各种符合各种逻辑关系的合并操作。

l  Concat

In [73]: df = pd.DataFrame(np.random.randn(10, 4))In [74]: dfOut[74]:           0         1         2         30 -0.548702  1.467327 -1.015962 -0.4830751  1.637550 -1.217659 -0.291519 -1.7455052 -0.263952  0.991460 -0.919069  0.2660463 -0.709661  1.669052  1.037882 -1.7057754 -0.919854 -0.042379  1.247642 -0.0099205  0.290213  0.495767  0.362949  1.5481066 -1.131345 -0.089329  0.337863 -0.9458677 -0.932132  1.956030  0.017587 -0.0166928 -0.575247  0.254161 -1.143704  0.2158979  1.193555 -0.077118 -0.408530 -0.862495# break it into piecesIn [75]: pieces = [df[:3], df[3:7], df[7:]]In [76]: pd.concat(pieces)Out[76]:           0         1         2         30 -0.548702  1.467327 -1.015962 -0.4830751  1.637550 -1.217659 -0.291519 -1.7455052 -0.263952  0.991460 -0.919069  0.2660463 -0.709661  1.669052  1.037882 -1.7057754 -0.919854 -0.042379  1.247642 -0.0099205  0.290213  0.495767  0.362949  1.5481066 -1.131345 -0.089329  0.337863 -0.9458677 -0.932132  1.956030  0.017587 -0.0166928 -0.575247  0.254161 -1.143704  0.2158979  1.193555 -0.077118 -0.408530 -0.862495

l  Join 类似于SQL类型的合并

In [77]: left = pd.DataFrame({'key': ['foo', 'foo'], 'lval': [1, 2]})In [78]: right = pd.DataFrame({'key': ['foo', 'foo'], 'rval': [4, 5]})In [79]: leftOut[79]:    key  lval0  foo     11  foo     2In [80]: rightOut[80]:    key  rval0  foo     41  foo     5In [81]: pd.merge(left, right, on='key')Out[81]:    key  lval  rval0  foo     1     41  foo     1     52  foo     2     43  foo     2     5

l  Append 将一行连接到一个DataFrame上

In [82]: df = pd.DataFrame(np.random.randn(8, 4), columns=['A','B','C','D'])In [83]: dfOut[83]:           A         B         C         D0  1.346061  1.511763  1.627081 -0.9905821 -0.441652  1.211526  0.268520  0.0245802 -1.577585  0.396823 -0.105381 -0.5325323  1.453749  1.208843 -0.080952 -0.2646104 -0.727965 -0.589346  0.339969 -0.6932055 -0.339355  0.593616  0.884345  1.5914316  0.141809  0.220390  0.435589  0.1924517 -0.096701  0.803351  1.715071 -0.708758In [84]: s = df.iloc[3]In [85]: df.append(s, ignore_index=True)Out[85]:           A         B         C         D0  1.346061  1.511763  1.627081 -0.9905821 -0.441652  1.211526  0.268520  0.0245802 -1.577585  0.396823 -0.105381 -0.5325323  1.453749  1.208843 -0.080952 -0.2646104 -0.727965 -0.589346  0.339969 -0.6932055 -0.339355  0.593616  0.884345  1.5914316  0.141809  0.220390  0.435589  0.1924517 -0.096701  0.803351  1.715071 -0.7087588  1.453749  1.208843 -0.080952 -0.264610

七、分组

对于”group by”操作，我们通常是指以下一个或多个操作步骤：

l  （Splitting）按照一些规则将数据分为不同的组；

l  （Applying）对于每组数据分别执行一个函数；

l  （Combining）将结果组合到一个数据结构中；

In [86]: df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',   ....:                           'foo', 'bar', 'foo', 'foo'],   ....:                    'B' : ['one', 'one', 'two', 'three',   ....:                           'two', 'two', 'one', 'three'],   ....:                    'C' : np.random.randn(8),   ....:                    'D' : np.random.randn(8)})   ....: In [87]: dfOut[87]:      A      B         C         D0  foo    one -1.202872 -0.0552241  bar    one -1.814470  2.3959852  foo    two  1.018601  1.5528253  bar  three -0.595447  0.1665994  foo    two  1.395433  0.0476095  bar    two -0.392670 -0.1364736  foo    one  0.007207 -0.5617577  foo  three  1.928123 -1.623033

1、  分组并对每个分组执行sum函数：

In [88]: df.groupby('A').sum()Out[88]:             C        DA                     bar -2.802588  2.42611foo  3.146492 -0.63958

2、  通过多个列进行分组形成一个层次索引，然后执行函数：

In [89]: df.groupby(['A','B']).sum()Out[89]:                   C         DA   B                        bar one   -1.814470  2.395985    three -0.595447  0.166599    two   -0.392670 -0.136473foo one   -1.195665 -0.616981    three  1.928123 -1.623033    two    2.414034  1.600434

八、重塑（Reshaping）

l  Stack

In [90]: tuples = list(zip(*[['bar', 'bar', 'baz', 'baz',   ....:                      'foo', 'foo', 'qux', 'qux'],   ....:                     ['one', 'two', 'one', 'two',   ....:                      'one', 'two', 'one', 'two']]))   ....: In [91]: index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])In [92]: df = pd.DataFrame(np.random.randn(8, 2), index=index, columns=['A', 'B'])In [93]: df2 = df[:4]In [94]: df2Out[94]:                      A         Bfirst second                    bar   one     0.029399 -0.542108      two     0.282696 -0.087302baz   one    -1.575170  1.771208      two     0.816482  1.100230

stack()方法在DataFrame列的层次上压缩

In [95]: stacked = df2.stack()In [96]: stackedOut[96]: first  second   bar    one     A    0.029399               B   -0.542108       two     A    0.282696               B   -0.087302baz    one     A   -1.575170               B    1.771208       two     A    0.816482               B    1.100230dtype: float64

整合DataFrame或者Series（MultiIndex作为索引），stack()的逆运算是isunstack(),默认情况下unStack上一次的情况。

In [97]: stacked.unstack()Out[97]:                      A         Bfirst second                    bar   one     0.029399 -0.542108      two     0.282696 -0.087302baz   one    -1.575170  1.771208      two     0.816482  1.100230In [98]: stacked.unstack(1)Out[98]: second        one       twofirst                      bar   A  0.029399  0.282696      B -0.542108 -0.087302baz   A -1.575170  0.816482      B  1.771208  1.100230In [99]: stacked.unstack(0)Out[99]: first          bar       bazsecond                      one    A  0.029399 -1.575170       B -0.542108  1.771208two    A  0.282696  0.816482       B -0.087302  1.100230

l  数据透视表

In [100]: df = pd.DataFrame({'A' : ['one', 'one', 'two', 'three'] * 3,   .....:                    'B' : ['A', 'B', 'C'] * 4,   .....:                    'C' : ['foo', 'foo', 'foo', 'bar', 'bar', 'bar'] * 2,   .....:                    'D' : np.random.randn(12),   .....:                    'E' : np.random.randn(12)})   .....: In [101]: dfOut[101]:         A  B    C         D         E0     one  A  foo  1.418757 -0.1796661     one  B  foo -1.879024  1.2918362     two  C  foo  0.536826 -0.0096143   three  A  bar  1.006160  0.3921494     one  B  bar -0.029716  0.2645995     one  C  bar -1.146178 -0.0574096     two  A  foo  0.100900 -1.4256387   three  B  foo -1.035018  1.0240988     one  C  foo  0.314665 -0.1060629     one  A  bar -0.773723  1.82437510    two  B  bar -1.170653  0.59597411  three  C  bar  0.648740  1.167115

可以从这个数据中轻松的生成数据透视表：

In [102]: pd.pivot_table(df, values='D', index=['A', 'B'], columns=['C'])Out[102]: C             bar       fooA     B                    one   A -0.773723  1.418757      B -0.029716 -1.879024      C -1.146178  0.314665three A  1.006160       NaN      B       NaN -1.035018      C  0.648740       NaNtwo   A       NaN  0.100900      B -1.170653       NaN      C       NaN  0.536826

九、时间序列

Pandas在对频率转换进行重新采样时拥有简单、强大且高效的功能（如将按秒采样的数据转换为按5分钟为单位进行采样的数据）。这种操作在金融领域非常常见。

In [103]: rng = pd.date_range('1/1/2012', periods=100, freq='S')In [104]: ts = pd.Series(np.random.randint(0, 500, len(rng)), index=rng)In [105]: ts.resample('5Min').sum()Out[105]: 2012-01-01    25083Freq: 5T, dtype: int64

1、  时区表示：

In [106]: rng = pd.date_range('3/6/2012 00:00', periods=5, freq='D')In [107]: ts = pd.Series(np.random.randn(len(rng)), rng)In [108]: tsOut[108]: 2012-03-06    0.4640002012-03-07    0.2273712012-03-08   -0.4969222012-03-09    0.3063892012-03-10   -2.290613Freq: D, dtype: float64In [109]: ts_utc = ts.tz_localize('UTC')In [110]: ts_utcOut[110]: 2012-03-06 00:00:00+00:00    0.4640002012-03-07 00:00:00+00:00    0.2273712012-03-08 00:00:00+00:00   -0.4969222012-03-09 00:00:00+00:00    0.3063892012-03-10 00:00:00+00:00   -2.290613Freq: D, dtype: float64

2、  时区转换：

In [111]: ts_utc.tz_convert('US/Eastern')Out[111]: 2012-03-05 19:00:00-05:00    0.4640002012-03-06 19:00:00-05:00    0.2273712012-03-07 19:00:00-05:00   -0.4969222012-03-08 19:00:00-05:00    0.3063892012-03-09 19:00:00-05:00   -2.290613Freq: D, dtype: float64

3、  时间跨度转换：

In [112]: rng = pd.date_range('1/1/2012', periods=5, freq='M')In [113]: ts = pd.Series(np.random.randn(len(rng)), index=rng)In [114]: tsOut[114]: 2012-01-31   -1.1346232012-02-29   -1.5618192012-03-31   -0.2608382012-04-30    0.2819572012-05-31    1.523962Freq: M, dtype: float64In [115]: ps = ts.to_period()In [116]: psOut[116]: 2012-01   -1.1346232012-02   -1.5618192012-03   -0.2608382012-04    0.2819572012-05    1.523962Freq: M, dtype: float64In [117]: ps.to_timestamp()Out[117]: 2012-01-01   -1.1346232012-02-01   -1.5618192012-03-01   -0.2608382012-04-01    0.2819572012-05-01    1.523962Freq: MS, dtype: float64

4、  时期和时间戳之间的转换使得可以使用一些方便的算术函数。

In [118]: prng = pd.period_range('1990Q1', '2000Q4', freq='Q-NOV')In [119]: ts = pd.Series(np.random.randn(len(prng)), prng)In [120]: ts.index = (prng.asfreq('M', 'e') + 1).asfreq('H', 's') + 9In [121]: ts.head()Out[121]: 1990-03-01 09:00   -0.9029371990-06-01 09:00    0.0681591990-09-01 09:00   -0.0578731990-12-01 09:00   -0.3682041991-03-01 09:00   -1.144073Freq: H, dtype: float64

十、Categorical

从0.15版本开始，pandas可以在DataFrame中支持Categorical类型的数据。

In [122]: df = pd.DataFrame({"id":[1,2,3,4,5,6], "raw_grade":['a', 'b', 'b', 'a', 'a', 'e']})

1、  将原始的grade转换为Categorical数据类型：

In [123]: df["grade"] = df["raw_grade"].astype("category")In [124]: df["grade"]Out[124]: 0    a1    b2    b3    a4    a5    eName: grade, dtype: categoryCategories (3, object): [a, b, e]

2、  将Categorical类型数据重命名为更有意义的名称：

In [125]: df["grade"].cat.categories = ["very good", "good", "very bad"]

3、  对类别进行重新排序，增加缺失的类别：

In [126]: df["grade"] = df["grade"].cat.set_categories(["very bad", "bad", "medium", "good", "very good"])In [127]: df["grade"]Out[127]: 0    very good1         good2         good3    very good4    very good5     very badName: grade, dtype: categoryCategories (5, object): [very bad, bad, medium, good, very good]

4、  排序是按照Categorical的顺序进行的而不是按照字典顺序进行：

In [128]: df.sort_values(by="grade")Out[128]:    id raw_grade      grade5   6         e   very bad1   2         b       good2   3         b       good0   1         a  very good3   4         a  very good4   5         a  very good

5、  对Categorical列进行排序时存在空的类别：

In [129]: df.groupby("grade").size()Out[129]: gradevery bad     1bad          0medium       0good         2very good    3dtype: int64

十一、画图

In [130]: ts = pd.Series(np.random.randn(1000), index=pd.date_range('1/1/2000', periods=1000))In [131]: ts = ts.cumsum()In [132]: ts.plot()Out[132]: <matplotlib.axes._subplots.AxesSubplot at 0x10efd5a90>

对于DataFrame来说，plot是一种将所有列及其标签进行绘制的简便方法：

In [133]: df = pd.DataFrame(np.random.randn(1000, 4), index=ts.index,   .....:                   columns=['A', 'B', 'C', 'D'])   .....: In [134]: df = df.cumsum()In [135]: plt.figure(); df.plot(); plt.legend(loc='best')Out[135]: <matplotlib.legend.Legend at 0x112854d90>

十二、导入和保存数据

l  CSV

1、  写入csv文件：

In [136]: df.to_csv('foo.csv')

2、  从csv文件中读取：

In [137]: pd.read_csv('foo.csv')Out[137]:      Unnamed: 0          A          B         C          D0    2000-01-01   0.266457  -0.399641 -0.219582   1.1868601    2000-01-02  -1.170732  -0.345873  1.653061  -0.2829532    2000-01-03  -1.734933   0.530468  2.060811  -0.5155363    2000-01-04  -1.555121   1.452620  0.239859  -1.1568964    2000-01-05   0.578117   0.511371  0.103552  -2.4282025    2000-01-06   0.478344   0.449933 -0.741620  -1.9624096    2000-01-07   1.235339  -0.091757 -1.543861  -1.084753..          ...        ...        ...       ...        ...993  2002-09-20 -10.628548  -9.153563 -7.883146  28.313940994  2002-09-21 -10.390377  -8.727491 -6.399645  30.914107995  2002-09-22  -8.985362  -8.485624 -4.669462  31.367740996  2002-09-23  -9.558560  -8.781216 -4.499815  30.518439997  2002-09-24  -9.902058  -9.340490 -4.386639  30.105593998  2002-09-25 -10.216020  -9.480682 -3.933802  29.758560999  2002-09-26 -11.856774 -10.671012 -3.216025  29.369368[1000 rows x 5 columns]

l  HDF5

1、  写入HDF5存储：

In [138]: df.to_hdf('foo.h5','df')

2、  从HDF5存储中读取：

In [139]: pd.read_hdf('foo.h5','df')Out[139]:                     A          B         C          D2000-01-01   0.266457  -0.399641 -0.219582   1.1868602000-01-02  -1.170732  -0.345873  1.653061  -0.2829532000-01-03  -1.734933   0.530468  2.060811  -0.5155362000-01-04  -1.555121   1.452620  0.239859  -1.1568962000-01-05   0.578117   0.511371  0.103552  -2.4282022000-01-06   0.478344   0.449933 -0.741620  -1.9624092000-01-07   1.235339  -0.091757 -1.543861  -1.084753...               ...        ...       ...        ...2002-09-20 -10.628548  -9.153563 -7.883146  28.3139402002-09-21 -10.390377  -8.727491 -6.399645  30.9141072002-09-22  -8.985362  -8.485624 -4.669462  31.3677402002-09-23  -9.558560  -8.781216 -4.499815  30.5184392002-09-24  -9.902058  -9.340490 -4.386639  30.1055932002-09-25 -10.216020  -9.480682 -3.933802  29.7585602002-09-26 -11.856774 -10.671012 -3.216025  29.369368[1000 rows x 4 columns]

l  Excel

1、  写入excel文件：

In [140]: df.to_excel('foo.xlsx', sheet_name='Sheet1')

2、  从excel文件中读取：

In [141]: pd.read_excel('foo.xlsx', 'Sheet1', index_col=None, na_values=['NA'])Out[141]:                     A          B         C          D2000-01-01   0.266457  -0.399641 -0.219582   1.1868602000-01-02  -1.170732  -0.345873  1.653061  -0.2829532000-01-03  -1.734933   0.530468  2.060811  -0.5155362000-01-04  -1.555121   1.452620  0.239859  -1.1568962000-01-05   0.578117   0.511371  0.103552  -2.4282022000-01-06   0.478344   0.449933 -0.741620  -1.9624092000-01-07   1.235339  -0.091757 -1.543861  -1.084753...               ...        ...       ...        ...2002-09-20 -10.628548  -9.153563 -7.883146  28.3139402002-09-21 -10.390377  -8.727491 -6.399645  30.9141072002-09-22  -8.985362  -8.485624 -4.669462  31.3677402002-09-23  -9.558560  -8.781216 -4.499815  30.5184392002-09-24  -9.902058  -9.340490 -4.386639  30.1055932002-09-25 -10.216020  -9.480682 -3.933802  29.7585602002-09-26 -11.856774 -10.671012 -3.216025  29.369368[1000 rows x 4 columns]

十三、Gotchas

如果你正在运行，你会看到这样一个异常：

>>> if pd.Series([False, True, False]):    print("I was true")Traceback    ...ValueError: The truth value of an array is ambiguous. Use a.empty, a.any() or a.all().