4.1 数据分析流程
探索性数据分析，Exploratory Data Analysis (EDA) ，通常不包括创建模型，但包括总结数据集的特征和可视化。
读取数据，使用.sample方法查看数据：
>>> import pandas as pd
>>> import numpy as np
>>> college = pd.read_csv("data/college.csv")
>>> college.sample(random_state=42)
INSTNM CITY ... MD_EARN_WNE_P10 GRAD_DEBT_MDN_SUPP
3649 Career P... San Antonio ... 20700 14977查看数据集的维度：
代码语言：javascript复制>>> college.shape
(7535, 27)用.info方法，查看每列的数据类型，非空值的数量，内存占用：
>>> college.info()
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 7535 entries, 0 to 7534
Data columns (total 27 columns):
# Column Non-Null Count Dtype
--- ------ -------------- -----
0 INSTNM 7535 non-null object
1 CITY 7535 non-null object
2 STABBR 7535 non-null object
3 HBCU 7164 non-null float64
4 MENONLY 7164 non-null float64
5 WOMENONLY 7164 non-null float64
6 RELAFFIL 7535 non-null int64
7 SATVRMID 1185 non-null float64
8 SATMTMID 1196 non-null float64
9 DISTANCEONLY 7164 non-null float64
10 UGDS 6874 non-null float64
11 UGDS_WHITE 6874 non-null float64
12 UGDS_BLACK 6874 non-null float64
13 UGDS_HISP 6874 non-null float64
14 UGDS_ASIAN 6874 non-null float64
15 UGDS_AIAN 6874 non-null float64
16 UGDS_NHPI 6874 non-null float64
17 UGDS_2MOR 6874 non-null float64
18 UGDS_NRA 6874 non-null float64
19 UGDS_UNKN 6874 non-null float64
20 PPTUG_EF 6853 non-null float64
21 CURROPER 7535 non-null int64
22 PCTPELL 6849 non-null float64
23 PCTFLOAN 6849 non-null float64
24 UG25ABV 6718 non-null float64
25 MD_EARN_WNE_P10 6413 non-null object
26 GRAD_DEBT_MDN_SUPP 7503 non-null object
dtypes: float64(20), int64(2), object(5)
memory usage: 1.6 MB查看数值列的情况，转置以读取更多输出：
代码语言：javascript复制>>> college.describe(include=[np.number]).T
count mean ... 75% max
HBCU 7164.0 0.014238 ... 0.000000 1.0
MENONLY 7164.0 0.009213 ... 0.000000 1.0
WOMENONLY 7164.0 0.005304 ... 0.000000 1.0
RELAFFIL 7535.0 0.190975 ... 0.000000 1.0
SATVRMID 1185.0 522.819409 ... 555.000000 765.0
... ... ... ... ... ...
PPTUG_EF 6853.0 0.226639 ... 0.376900 1.0
CURROPER 7535.0 0.923291 ... 1.000000 1.0
PCTPELL 6849.0 0.530643 ... 0.712900 1.0
PCTFLOAN 6849.0 0.522211 ... 0.745000 1.0
UG25ABV 6718.0 0.410021 ... 0.572275 1.0查看对象（字符串）列的情况：
代码语言：javascript复制>>> college.describe(include=[np.object]).T
count unique top freq
INSTNM 7535 7535 Academy ... 1
CITY 7535 2514 New York 87
STABBR 7535 59 CA 773
MD_EARN_W... 6413 598 PrivacyS... 822
GRAD_DEBT... 7503 2038 PrivacyS... 1510更多
在.describe方法中指定分位数：
>>> college.describe(
>>> include=[np.number],
... percentiles=[
... 0.01,
... 0.05,
... 0.10,
... 0.25,
... 0.5,
... 0.75,
... 0.9,
... 0.95,
... 0.99,
... ],
... ).T
count mean ... 99% max
HBCU 7164.0 0.014238 ... 1.000000 1.0
MENONLY 7164.0 0.009213 ... 0.000000 1.0
WOMENONLY 7164.0 0.005304 ... 0.000000 1.0
RELAFFIL 7535.0 0.190975 ... 1.000000 1.0
SATVRMID 1185.0 522.819409 ... 730.000000 765.0
... ... ... ... ... ...
PPTUG_EF 6853.0 0.226639 ... 0.946724 1.0
CURROPER 7535.0 0.923291 ... 1.000000 1.0
PCTPELL 6849.0 0.530643 ... 0.993908 1.0
PCTFLOAN 6849.0 0.522211 ... 0.986368 1.0
UG25ABV 6718.0 0.410021 ... 0.917383 1.04.2 数据字典
数据字典是个包含元数据和注释的表格，它的主要目的是解释列名。
college_data_dictionary.csv中包含了大学数据集的数据字典：
>>> pd.read_csv("data/college_data_dictionary.csv")
column_name description
0 INSTNM Institut...
1 CITY City Loc...
2 STABBR State Ab...
3 HBCU Historic...
4 MENONLY 0/1 Men ...
.. ... ...
22 PCTPELL Percent ...
23 PCTFLOAN Percent ...
24 UG25ABV Percent ...
25 MD_EARN_... Median E...
26 GRAD_DEB... Median d...4.3 通过改变数据类型降低内存
选取一些数据类型不同的列，以便观察内存占用：
代码语言：javascript复制>>> college = pd.read_csv("data/college.csv")
>>> different_cols = [
... "RELAFFIL",
... "SATMTMID",
... "CURROPER",
... "INSTNM",
... "STABBR",
... ]
>>> col2 = college.loc[:, different_cols]
>>> col2.head()
RELAFFIL SATMTMID ... INSTNM STABBR
0 0 420.0 ... Alabama ... AL
1 0 565.0 ... Universi... AL
2 1 NaN ... Amridge ... AL
3 0 590.0 ... Universi... AL
4 0 430.0 ... Alabama ... AL检查每列的数据类型：
代码语言：javascript复制>>> col2.dtypes
RELAFFIL int64
SATMTMID float64
CURROPER int64
INSTNM object
STABBR object
dtype: object检查每列占用了多少内存：
代码语言：javascript复制>>> original_mem = col2.memory_usage(deep=True)
>>> original_mem
Index 128
RELAFFIL 60280
SATMTMID 60280
CURROPER 60280
INSTNM 660240
STABBR 444565
dtype: int64列RELAFFIL没有必要使用64位，使用.astype方法将其转换为8位：
>>> col2["RELAFFIL"] = col2["RELAFFIL"].astype(np.int8)再次检查数据类型：
代码语言：javascript复制>>> col2.dtypes
RELAFFIL int8
SATMTMID float64
CURROPER int64
INSTNM object
STABBR object
dtype: object 再次检查内存占用，可以看到RELAFFIL一列的内存大大降低：
>>> col2.memory_usage(deep=True)
Index 128
RELAFFIL 7535
SATMTMID 60280
CURROPER 60280
INSTNM 660240
STABBR 444565
dtype: int64要节省更多内存，如果基数（基数是唯一值的数量）低的话，可以将object数据改为category：
代码语言：javascript复制>>> col2.select_dtypes(include=["object"]).nunique()
INSTNM 7535
STABBR 59
dtype: int64STABBR列的基数低，不到原始数据的百分之一，可以将其做转换：
>>> col2["STABBR"] = col2["STABBR"].astype("category")
>>> col2.dtypes
RELAFFIL int8
SATMTMID float64
CURROPER int64
INSTNM object
STABBR category
dtype: object再次查看内存占用：
代码语言：javascript复制>>> new_mem = col2.memory_usage(deep=True)
>>> new_mem
Index 128
RELAFFIL 7535
SATMTMID 60280
CURROPER 60280
INSTNM 660699
STABBR 13576
dtype: int64 最后，比较内存优化前后的比例：
代码语言：javascript复制>>> new_mem / original_mem
Index 1.000000
RELAFFIL 0.125000
SATMTMID 1.000000
CURROPER 1.000000
INSTNM 1.000695
STABBR 0.030538
dtype: float64更多
改变列中的一个值，可以观察内存的变化：
代码语言：javascript复制>>> college.loc[0, "CURROPER"] = 10000000
>>> college.loc[0, "INSTNM"] = (
... college.loc[0, "INSTNM"] "a"
... )
>>> college[["CURROPER", "INSTNM"]].memory_usage(deep=True)
Index 80
CURROPER 60280
INSTNM 660804
dtype: int64如果整数列中的一个值变为缺失值，则该列会强制变为浮点类型：
代码语言：javascript复制>>> college["MENONLY"].dtype
dtype('float64')
>>> college["MENONLY"].astype(np.int8)
Traceback (most recent call last):
...
ValueError: Cannot convert non-finite values (NA or inf) to integer下面的四种方法的效果相同：
代码语言：javascript复制college.describe(include=['int64', 'float64']).T
college.describe(include=[np.int64, np.float64]).T
college.describe(include=['int', 'float']).T
college.describe(include=['number']).T数据类型字符串还可以用在.astype方法中：
>>> college.assign(
... MENONLY=college["MENONLY"].astype("float16"),
... RELAFFIL=college["RELAFFIL"].astype("int8"),
... )
INSTNM CITY ... MD_EARN_WNE_P10 GRAD_DEBT_MDN_SUPP
0 Alabama ... Normal ... 30300 33888
1 Universi... Birmingham ... 39700 21941.5
2 Amridge ... Montgomery ... 40100 23370
3 Universi... Huntsville ... 45500 24097
4 Alabama ... Montgomery ... 26600 33118.5
... ... ... ... ... ...
7530 SAE Inst... Emeryville ... NaN 9500
7531 Rasmusse... Overland... ... NaN 21163
7532 National... Highland... ... NaN 6333
7533 Bay Area... San Jose ... NaN PrivacyS...
7534 Excel Le... San Antonio ... NaN 12125查看RangeIndex和Int64Index两种索引类型的内存占用：
>>> college.index = pd.Int64Index(college.index)
>>> college.index.memory_usage() # 原先是80
602804.4 选择最大值中最小值
使用.nlargest和.nsmallest从排名前100的电影中，选择成本最低的5个电影：
>>> movie = pd.read_csv("data/movie.csv")
>>> movie2 = movie[["movie_title", "imdb_score", "budget"]]
>>> movie2.head()
movie_title imdb_score budget
0 Avatar 7.9 237000000.0
1 Pirates ... 7.1 300000000.0
2 Spectre 6.8 245000000.0
3 The Dark... 8.5 250000000.0
4 Star War... 7.1 NaN选择imdb_score得分前100的电影：
>>> movie2.nlargest(100, "imdb_score").head()
movie_title imdb_score budget
movie_title imdb_score budget
2725 Towering Inferno 9.5 NaN
1920 The Shawshank Redemption 9.3 25000000.0
3402 The Godfather 9.2 6000000.0
2779 Dekalog 9.1 NaN
4312 Kickboxer: Vengeance 9.1 17000000.0选择前百名电影中，成本最低的5个电影：
代码语言：javascript复制>>> (
... movie2.nlargest(100, "imdb_score").nsmallest(
... 5, "budget"
... )
... )
movie_title imdb_score budget
4804 Butterfly Girl 8.7 180000.0
4801 Children of Heaven 8.5 180000.0
4706 12 Angry Men 8.9 350000.0
4550 A Separation 8.4 500000.0
4636 The Other Dream Team 8.4 500000.04.5 使用排序选择每组中最大值
在本节中，我们尝试找到每年评分最高的电影。
读取电影数据集，只保留其中的三个列：movie_title、title_year、imdb_score：
>>> movie = pd.read_csv("data/movie.csv")
>>> movie[["movie_title", "title_year", "imdb_score"]]
movie_title ...
0 Avatar ...
1 Pirates of the Caribbean: At World's End ...
2 Spectre ...
3 The Dark Knight Rises ...
4 Star Wars: Episode VII - The Force Awakens ...
... ... ...
4911 Signed Sealed Delivered ...
4912 The Following ...
4913 A Plague So Pleasant ...
4914 Shanghai Calling ...
4915 My Date with Drew ...按照title_year进行从小到大排列：
>>> (
... movie[
... ["movie_title", "title_year", "imdb_score"]
... ].sort_values("title_year", ascending=True)
... )
movie_title ...
4695 Intolerance: Love's Struggle Throughout the Ages ...
4833 Over the Hill to the Poorhouse ...
4767 The Big Parade ...
2694 Metropolis ...
4697 The Broadway Melody ...
... ... ...
4683 Heroes ...
4688 Home Movies ...
4704 Revolution ...
4752 Happy Valley ...
4912 The Following ...同时对两列进行排列：
代码语言：javascript复制>>> (
... movie[
... ["movie_title", "title_year", "imdb_score"]
... ].sort_values(
... ["title_year", "imdb_score"], ascending=False
... )
... )
movie_title title_year imdb_score
4312 Kickboxer: Vengeance 2016.0 9.1
4277 A Beginner's Guide to Snuff 2016.0 8.7
3798 Airlift 2016.0 8.5
27 Captain America: Civil War 2016.0 8.2
98 Godzilla Resurgence 2016.0 8.2
... ... ... ...
1391 Rush Hour NaN 5.8
4031 Creature NaN 5.0
2165 Meet the Browns NaN 3.5
3246 The Bold and the Beautiful NaN 3.5
2119 The Bachelor NaN 2.9使用.drop_duplicates方法，只保留每年的第一行数据：
>>> (
... movie[["movie_title", "title_year", "imdb_score"]]
... .sort_values(
... ["title_year", "imdb_score"], ascending=False
... )
... .drop_duplicates(subset="title_year")
... )
movie_title title_year imdb_score
4312 Kickboxe... 2016.0 9.1
3745 Running ... 2015.0 8.6
4369 Queen of... 2014.0 8.7
3935 Batman: ... 2013.0 8.4
3 The Dark... 2012.0 8.5
... ... ... ...
2694 Metropolis 1927.0 8.3
4767 The Big ... 1925.0 8.3
4833 Over the... 1920.0 4.8
4695 Intolera... 1916.0 8.0
2725 Towering... NaN 9.5更多
还可以使用.groupby方法，实现同样的目标：
>>> (
... movie[["movie_title", "title_year", "imdb_score"]]
... .groupby("title_year", as_index=False)
... .apply(
... lambda df:df.sort_values(
... "imdb_score", ascending=False
... ).head(1)
... )
... .droplevel(0)
... .sort_values("title_year", ascending=False)
... )
movie_title title_year imdb_score
90 4312 Kickboxe... 2016.0 9.1
89 3745 Running ... 2015.0 8.6
88 4369 Queen of... 2014.0 8.7
87 3935 Batman: ... 2013.0 8.4
86 3 The Dark... 2012.0 8.5
... ... ... ...
4 4555 Pandora'... 1929.0 8.0
3 2694 Metropolis 1927.0 8.3
2 4767 The Big ... 1925.0 8.3
1 4833 Over the... 1920.0 4.8
0 4695 Intolera... 1916.0 8.0参数ascending可以传入一组布尔值，对列分别进行排列：
>>> (
... movie[
... [
... "movie_title",
... "title_year",
... "content_rating",
... "budget",
... ]
... ]
... .sort_values(
... ["title_year", "content_rating", "budget"],
... ascending=[False, False, True],
... )
... .drop_duplicates(
... subset=["title_year", "content_rating"]
... )
... )
movie_title title_year content_rating budget
4026 Compadres 2016.0 R 3000000.0
4658 Fight to... 2016.0 PG-13 150000.0
4661 Rodeo Girl 2016.0 PG 500000.0
3252 The Wailing 2016.0 Not Rated NaN
4659 Alleluia... 2016.0 NaN 500000.0
... ... ... ... ...
2558 Lilyhammer NaN TV-MA 34000000.0
807 Sabrina,... NaN TV-G 3000000.0
848 Stargate... NaN TV-14 1400000.0
2436 Carlos NaN Not Rated NaN
2119 The Bach... NaN NaN 3000000.04.6 使用sort_values复制n个最大值
使用nlargest和. nsmallest实现4.5中同样的目标：
>>> movie = pd.read_csv("data/movie.csv")
>>> (
... movie[["movie_title", "imdb_score", "budget"]]
... .nlargest(100, "imdb_score")
... .nsmallest(5, "budget")
... )
movie_title imdb_score budget
4804 Butterfly Girl 8.7 180000.0
4801 Children of Heaven 8.5 180000.0
4706 12 Angry Men 8.9 350000.0
4550 A Separation 8.4 500000.0
4636 The Other Dream Team 8.4 500000.0使用.sort_values实现提取imdb_score最高的100部电影：
>>> (
... movie[["movie_title", "imdb_score", "budget"]]
... .sort_values("imdb_score", ascending=False)
... .head(100)
... )
movie_title imdb_score budget
2725 Towering... 9.5 NaN
1920 The Shaw... 9.3 25000000.0
3402 The Godf... 9.2 6000000.0
2779 Dekalog 9.1 NaN
4312 Kickboxe... 9.1 17000000.0
... ... ... ...
3799 Anne of ... 8.4 NaN
3777 Requiem ... 8.4 4500000.0
3935 Batman: ... 8.4 3500000.0
4636 The Othe... 8.4 500000.0
2455 Aliens 8.4 18500000.0紧接着上一步操作，提取其中成本最低的5部电影：
代码语言：javascript复制>>> (
... movie[["movie_title", "imdb_score", "budget"]]
... .sort_values("imdb_score", ascending=False)
... .head(100)
... .sort_values("budget")
... .head(5)
... )
movie_title imdb_score budget
4815 A Charlie Brown Christmas 8.4 150000.0
4801 Children of Heaven 8.5 180000.0
4804 Butterfly Girl 8.7 180000.0
4706 12 Angry Men 8.9 350000.0
4636 The Other Dream Team 8.4 500000.0更多
因为8.4分的电影比较多，.nlargest和.sort_values略有不同，导致得分前百的电影的DataFrame并不相同。如果在.sort_values中传入kind='mergsort'，两者的结果就相同了。
>>> (
... movie[["movie_title", "imdb_score", "budget"]]
... .nlargest(100, "imdb_score")
... .tail()
... )
movie_title imdb_score budget
4023 Oldboy 8.4 3000000.0
4163 To Kill a Mockingbird 8.4 2000000.0
4395 Reservoir Dogs 8.4 1200000.0
4550 A Separation 8.4 500000.0
4636 The Other Dream Team 8.4 500000.0
>>> (
... movie[["movie_title", "imdb_score", "budget"]]
... .sort_values("imdb_score", ascending=False)
... .head(100)
... .tail()
... )
movie_title imdb_score budget
3799 Anne of ... 8.4 NaN
3777 Requiem ... 8.4 4500000.0
3935 Batman: ... 8.4 3500000.0
4636 The Othe... 8.4 500000.0
2455 Aliens 8.4 18500000.04.7 计算跟踪止损单
止损单是股票交易中常用的术语。如果用100美元每股购买了股票，如果是90%止损，则止损单的价格是90美元。
如果股票涨到了120美元，则止损单价格变为了108美元。如果股票降到了110美元，止损单价格仍然是108美元。只有股票上涨时，止损单价格才上涨。
本节需要安装第三方库pandas-datareader，使用conda install pandas-datareader或pip install pandas-datareader进行安装。可能还需要安装requests_cache。
这里使用的的是特斯拉的股票（TSLA），假设在2017开盘日购买了股票：
代码语言：javascript复制>>> import datetime
>>> import pandas_datareader.data as web
>>> import requests_cache
>>> session = requests_cache.CachedSession(
... cache_name="cache",
... backend="sqlite",
... expire_after=datetime.timedelta(days=90),
... )
>>> tsla = web.DataReader(
... "tsla",
... data_source="yahoo",
... start="2017-1-1",
... session=session,
... )
>>> tsla.head(8)
High Low ... Volume Adj Close
Date ...
2017-01-03 220.330002 210.960007 ... 5923300 216.990005
2017-01-04 228.000000 214.309998 ... 11213500 226.990005
2017-01-05 227.479996 221.949997 ... 5911700 226.750000
2017-01-06 230.309998 225.449997 ... 5527900 229.009995
2017-01-09 231.919998 228.000000 ... 3979500 231.279999
2017-01-10 232.000000 226.889999 ... 3660000 229.869995
2017-01-11 229.979996 226.679993 ... 3650800 229.729996
2017-01-12 230.699997 225.580002 ... 3790200 229.589996简单起见，使用的是每个交易日的收盘价：
代码语言：javascript复制>>> tsla_close = tsla["Close"]使用.cummax方法跟踪最高收盘价：
>>> tsla_cummax = tsla_close.cummax()
>>> tsla_cummax.head()
Date
2017-01-03 216.990005
2017-01-04 226.990005
2017-01-05 226.990005
2017-01-06 229.009995
2017-01-09 231.279999
Name: Close, dtype: float64乘以0.9就能得到止损单价格了：
代码语言：javascript复制>>> (tsla["Close"].cummax().mul(0.9).head())
Date
2017-01-03 195.291005
2017-01-04 204.291005
2017-01-05 204.291005
2017-01-06 206.108995
2017-01-09 208.151999
Name: Close, dtype: float64更多
和cummax相对，.cummin可以追踪最低值：
weight.cummin() * 1.05
