Python - pandas 之常用代码块[转]

原文：40 Useful Pandas Snippets Pandas snippets that come in handy in data analysis work - 2022.04.20

1. 过滤列数据

Filter columns.

只需要数据集的几列，如：

pd.read_csv("data.csv", usecols=["date", "price"])

2. 读取时解析日期

Parse dates on read.

pd.read_csv("data.csv", parse_dates=["date"])

3. 指定数据类型

Specify Data Types.

读取数据时，设置数据类型分类，以节省内存.

pd.read_csv("data.csv", dtype={"house_type": "category"})

4. 设置索引

Set index.

设置索引，尤其对于时间序列比较有用.

pd.read_csv("data.csv", index_col="date")

5. 读取的行数

No. of rows to read.

只读取数据集中的部分数据.

#100 行数据
pd.read_csv("data.csv", nrows=100)

6. 跳过行

Skip rows.

跳过某些行的数据，

#跳过第1行和第5行
pd.read_csv("data.csv", skiprows=[1, 5])

#跳过前100行
pd.read_csv("data.csv", skiprows=100)

#跳过 90% 的行
pd.read_csv("data.csv", skiprows=lambda x: x > 0 and np.random.rand() > 0.1)

7. 指定 NA 值

Specify NA Values.

如果数据的值有NA，即，如是 ? 等，将其设置为读取，以便以后不用再转换.

pd.read_csv("data.csv", na_values=["?"])

8. 设定布尔值

Setting boolean values.

如果某一列数据的格式是 Yes 和 No的形式，

pd.read_csv("data.csv", true_values=["yes"], false_values=["no"])

9. 读取多个文件

Read from multiple files.

如果数据被存储在多个文件里，

import glob
import os

files = glob.glob("file_*.csv")

result = pd.concat([pd.read_csv(f) for f in files], ignore_index=True)

10. 复制粘贴数据到DataFrames

Copy and Paste into Data Frames.

从剪切板复制粘贴数据，

 df = pd.read_clipboard()

11. 从PDF读取表格数据

Read tables from PDF files.

# pip install tabula-py

from tabula import read_pdf
# Read pdf into list of DataFrame
df = read_pdf('test.pdf', pages='all')

12. 探索性数据分析

EDA，Exploratory Data Analysis，探索性数据分析

EDA cheat.

想要可视化数据，但不想写plot代码，可以采用 pandas-profiling，仅需要一行代码，

# pip install pandas-profiling

import pandas_profiling

df = pd.read_csv("data.csv")

profile = df.profile_report(title="Pandas Profiling Report")
profile.to_file(output_file="output.html")

13. 通过 dtype 过滤列

dtype，Data Types. （list of dtypes for pandas）.

Filter columns by dtype.

# selecting
df.select_dtypes(include="number")
df.select_dtypes(include=["category", "datetime"])

# exluding
df.select_dtypes(exclude="object")

14. Infer dtype

df.infer_objects().dtypes

15. Downcasting

pd.to_numeric(df.numeric_col, downcast="integer") # smallest signed int dtype
pd.to_numeric(df.numeric_col, downcast="float")  # smallest float dtype

16. 手工转换

Manual conversion.

如果数据中有 NaN 值，error="coerce" 能够避免报错.

同时，可以采用 .fillna 将 NA 值填充为合理的值.

# apply to whole data frame
df = df.apply(pd.to_numeric, errors="coerce")

# apply to specific columns
pd.to_numeric(df.numeric_column, errors="coerce")

# filling NA values with zero
pd.to_numeric(df.numeric_column, errors="coerce").fillna(0)

17. 一次转换全部

Convert all at once.

df = df.astype(
    {
        "date": "datetime64[ns]",
        "price": "int",
        "is_weekend": "bool",
        "status": "category",
    }
)

18. 列重命名

Renaming columns.

df = df.rename({"PRICE": "price", "Date (mm/dd/yyyy)": "date"}, axis=1)

19. 增加前缀和后缀

Add suffix and prefix.

df.add_prefix("pre_")
df.add_suffix("_suf")

20. 创建新列

Create new columns.

# create new column of Fahrenheit values from Celcius
df.assign(temp_f=lambda x: x.temp_c * 9 / 5 + 32)

21. 指定位置插入列

Insert columns at specific positions.

random_col = np.random.randint(10, size=len(df))

df.insert(3, 'random_col', random_col) # inserts at third column

22. if-then-else

df["logic"] = np.where(df["price"] > 5, "high", "low")

23. Dropping columns

df.drop('col1', axis=1, inplace=True)
df = df.drop(['col1','col2'], axis=1)
s = df.pop('col')
del df['col']
df.drop(df.columns[0], inplace=True)

24. 字符串列名操作

Columns names

# on column names
df.columns = df.columns.str.lower()
df.columns = df.columns.str.replace(' ', '_')

25. 字符串Contains

df['name'].str.contains("John")

df['phone_num'].str.contains('...-...-....', regex=True)  # regex

df['email'].str.contains('gmail')

26. 字符串findall

pattern = '([A-Z0-9._%+-]+)@([A-Z0-9.-]+)\\.([A-Z]{2,4})'

df['email'].str.findall(pattern, flags=re.IGNORECASE)

27. 缺失值检查

Missing values Checking

def missing_vals(df):
    """prints out columns with perc of missing values"""
    missing = [
        (df.columns[idx], perc)
        for idx, perc in enumerate(df.isna().mean() * 100)
        if perc > 0
    ]

    if len(missing) == 0:
        return "no missing values"
        

    # sort desc by perc
    missing.sort(key=lambda x: x[1], reverse=True)

    print(f"There are a total of {len(missing)} variables with missing values\n")

    for tup in missing:
        print(str.ljust(f"{tup[0]:<20} => {round(tup[1], 3)}%", 1))

#
missing_vals(df)

如：

There are a total of 16 variables with missing values

PoolQC               => 100.0%
Alley                => 94.0%
MiscFeature          => 91.0%
Fence                => 77.0%
FireplaceQu          => 54.0%
LotFrontage          => 14.0%
GarageType           => 6.0%
GarageYrBlt          => 6.0%
GarageFinish         => 6.0%
GarageQual           => 6.0%
GarageCond           => 6.0%
BsmtQual             => 3.0%
BsmtCond             => 3.0%
BsmtExposure         => 3.0%
BsmtFinType1         => 3.0%
BsmtFinType2         => 3.0%

28. 缺失值处理

Dealing with missing values.

# drop 
df.dropna(axis=0)
df.dropna(axis=1)

# impute
df.fillna(0)
df.fillna(method="ffill")
df.fillna(method='bfill')

# replace
df.replace( -999, np.nan)
df.replace("?", np.nan)

# interpolate
ts.interpolate() # time series
df.interpolate() # fill all consecutive values forward
df.interpolate(limit=1) # fill one consecutive value forward
df.interpolate(limit=1, limit_direction="backward")
df.interpolate(limit_direction="both")

Calculations with missing data

29. 日期操作之时间

# from today
date.today() + datetime.timedelta(hours=30)
date.today() + datetime.timedelta(days=30)
date.today() + datetime.timedelta(weeks=30)

# ago
date.today() - datetime.timedelta(days=365)

30. 日期操作之两个时间点之间过滤

Filter between two dates.

df[(df["Date"] > "2015-01-01") & (df["Date"] < "2017-01-01")]

31. 根据day/month/year 过滤

# filter by single day
df[df["Date"].dt.strftime("%Y-%m-%d") == "2017-03-01"]

# filter by single month
df[df["Date"].dt.strftime("%m") == "12"]

# filter by single year
df[df["Date"].dt.strftime("%Y") == "2017"]

32. 数据格式化

format_dict = {
    "Date": "{:%d/%m/%y}",
    "Open": "${:.2f}",
    "Close": "${:.2f}",
    "Volume": "{:,}",
}

#
df.style.format(format_dict)

33. 颜色填充

(
    df.style.format(format_dict)
    .hide_index()
    .highlight_min(["Open"], color="red")
    .highlight_max(["Open"], color="green")
    .background_gradient(subset="Close", cmap="Greens")
    .bar('Volume', color='lightblue', align='zero')
    .set_caption('Tesla Stock Prices in 2017')
)

Table Visualization

34. 获取列的最大最小值 id

df['col'].idxmin()
df['col'].idxmax()

35. dataframe 函数处理

df.applymap(lambda x: np.log(x))

36. 随机打乱数据

df.sample(frac=1, random_state=7).reset_index(drop=True)

37. 百分比变化

Percent change，百分比变化。对时间序列有用.

如: price of BTC over 3 days [30000, 33000, 31000] -> [NaN, 0.1, -0.06]

df['col_name'].pct_change()

38. 检查 dataframe 的内存使用量

df.memory_usage().sum() / (1024**2) #converting to MB

39. 将list值分解为多行

Explode list values to multiple rows.

df.explode("col_name").reset_index(drop=True)

40. 将较少的数据归类为 Others

Convert smaller categories to “Others”

subclass = df.MSSubClass
subclass.value_counts()


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