Pandas类型操作

皮大大

发布于 2023-08-25 11:39:34

6530

pandas数据类型操作

介绍Pandas中3个常见的数据类型操作方法：

to_numeric
astype
to_datetime
select_dtypes

import pandas as pd
import numpy as np

Pandas字段类型

to_numeric()

官网地址：https://pandas.pydata.org/docs/reference/api/pandas.to_numeric.html

pandas.to_numeric(arg,  # scalar, list, tuple, 1-d array, or Series
                  errors='raise',  # ‘ignore’, ‘raise’, ‘coerce’；默认是raise
                  downcast=None)

errors的3种取值情况：

ignore：无效解析时直接返回输入
raise：无效解析引发异常
coerce：无效解析设置为NaN

downcast的使用：

字符串形式，默认是None，可以是‘integer’, ‘signed’, ‘unsigned’, or ‘float’
如果不是None，且已经转化成了某个数值型，才会向下个等级的数值类型转化
不同的数值类型
- 有符号整型：integer or signed，最小等级为np.int8
- 无符号整型：unsigned，最小等级为np.uint8
- 浮点型: 最小等级为np.float32

案例：假“数值型”

s = pd.Series(["2.0", '1', -3, 5.0])  # 数值（类似）
s

0    2.0
1      1
2     -3
3    5.0
dtype: object

默认是object类型，也就是字符串。下面转成数值型：

# 1、默认转成float64

pd.to_numeric(s)

0    2.0
1    1.0
2   -3.0
3    5.0
dtype: float64

# 2、指定类型

pd.to_numeric(s, downcast="integer")

0    2
1    1
2   -3
3    5
dtype: int8

# 3、指定类型

pd.to_numeric(s, downcast="signed")

0    2
1    1
2   -3
3    5
dtype: int8

# 4、指定类型

pd.to_numeric(s, downcast="unsigned")

0    2.0
1    1.0
2   -3.0
3    5.0
dtype: float64

# 5、指定类型

pd.to_numeric(s, downcast="float")

0    2.0
1    1.0
2   -3.0
3    5.0
dtype: float32

案例2：数值+字符串

s1 = pd.Series(["2.0", 'pandas', -3, 5.0])  # 数值+字符串
s1

0       2.0
1    pandas
2        -3
3       5.0
dtype: object

# pd.to_numeric(s1)   # 默认是会抛出异常

# 忽略异常

pd.to_numeric(s1, errors="ignore")

0       2.0
1    pandas
2        -3
3       5.0
dtype: object

# pd.to_numeric(s1, errors="raise")   # 无效解析引发异常

# 无效解析设置为None

pd.to_numeric(s1, errors="coerce")

0    2.0
1    NaN
2   -3.0
3    5.0
dtype: float64

# 无效解析设置为None

pd.to_numeric(s1, errors="coerce", downcast="float")

0    2.0
1    NaN
2   -3.0
3    5.0
dtype: float32

# 无效解析设置为None，最后用0代替

pd.to_numeric(s1, errors="coerce").fillna(0)

0    2.0
1    0.0
2   -3.0
3    5.0
dtype: float64

案例3-数值型

s2 = pd.Series([1,2.0,3.0], dtype="float64")
s2

0    1.0
1    2.0
2    3.0
dtype: float64

s3 = pd.to_numeric(s2, downcast="float")
s3

0    1.0
1    2.0
2    3.0
dtype: float32

s4 = pd.to_numeric(s2, downcast="integer")
s4

0    1
1    2
2    3
dtype: int8

类型转化的优势之一：节省内存资源。比较上面3种不同数值类型下的数据所占内存大小：

print("memory of float64: ", s2.memory_usage())
print("memory of float32: ", s3.memory_usage())
print("memory of int8: ", s4.memory_usage())

memory of float64:  152
memory of float32:  140
memory of int8:  131

astype

另一种转化的方法：astype

s2

0    1.0
1    2.0
2    3.0
dtype: float64

s2.astype("float32")

0    1.0
1    2.0
2    3.0
dtype: float32

s2.astype("int64")

0    1
1    2
2    3
dtype: int64

s2.astype("int32")

0    1
1    2
2    3
dtype: int32

s2.astype("category")

/Applications/downloads/anaconda/anaconda3/lib/python3.7/site-packages/pandas/io/formats/format.py:1429: FutureWarning: Index.ravel returning ndarray is deprecated; in a future version this will return a view on self.
  for val, m in zip(values.ravel(), mask.ravel())





0    1.0
1    2.0
2    3.0
dtype: category
Categories (3, float64): [1.0, 2.0, 3.0]

to_datetime()

pandas.to_datetime(arg,
                   errors='raise',
                   dayfirst=False,
                   yearfirst=False,
                   utc=None,
                   format=None,
                   exact=True,
                   unit=None,
                   infer_datetime_format=False,
                   origin='unix',
                   cache=True)

df = pd.DataFrame({"Year":[2022,2021,2022],
                   "Month":[1,3,5],
                   "Day":["10","12","28"]
                  })
df

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	Year	Month	Day
0	2022	1	10
1	2021	3	12
2	2022	5	28

df.dtypes

Year      int64
Month     int64
Day      object
dtype: object

直接拼接会报错：字符串和数值型不能直接相加。

# df["Date"] = df["Year"] + df["Month"] + df["Day"]

df["Date"] = pd.to_datetime(df)
df

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	Year	Month	Day	Date
0	2022	1	10	2022-01-10
1	2021	3	12	2021-03-12
2	2022	5	28	2022-05-28

df.dtypes

Year              int64
Month             int64
Day              object
Date     datetime64[ns]
dtype: object

pd.to_datetime("10/2/21")  # 默认

Timestamp('2021-10-02 00:00:00')

pd.to_datetime("10-2-21")  # 默认

Timestamp('2021-10-02 00:00:00')

pd.to_datetime("10/2/21",dayfirst=True)

Timestamp('2021-02-10 00:00:00')

pd.to_datetime("10/2/21",yearfirst=True)

Timestamp('2010-02-21 00:00:00')

pd.to_datetime("22-01-21",dayfirst=True)

Timestamp('2021-01-22 00:00:00')

pd.to_datetime("22-01-21",yearfirst=True)

Timestamp('2022-01-21 00:00:00')

pd.to_datetime('20220107', format='%Y%m%d', errors='ignore')

Timestamp('2022-01-07 00:00:00')

pd.to_datetime('20220107112347', errors='ignore')

Timestamp('2022-01-07 11:23:47')

pd.to_datetime('20220107112233', format='%Y%m%d%H%M%S')

Timestamp('2022-01-07 11:22:33')

select_dtypes

筛选指定类型下的数据信息

df.dtypes

Year              int64
Month             int64
Day              object
Date     datetime64[ns]
dtype: object

df.select_dtypes(include=["int"])

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	Year	Month
0	2022	1
1	2021	3
2	2022	5

df.select_dtypes(include=["object"])

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	Day
0	10
1	12
2	28

df.select_dtypes(include=["O"])  # 效果同上

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	Day
0	10
1	12
2	28

# 排除object字段类型

df.select_dtypes(exclude=["object"])

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	Year	Month	Date
0	2022	1	2022-01-10
1	2021	3	2021-03-12
2	2022	5	2022-05-28

# 排除object + int字段类型

df.select_dtypes(exclude=["object","int"])

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