#头条创作挑战赛#

一、背景介绍

Pandas 在处理大数据(尤其是列比较多的场景)时，如果不做优化，内存占用还是很大的，下面通过一个实例来说明可以怎样优化

首先，生成一批 18 万的数据，每条数据 151 列

import pandas as pd
import numpy as np


def gen_big_data(csv_file: str, big_data_count=90000):
    chars = 'abcdefghijklmnopqrstuvwxyz'
    dates = pd.date_range(start='2020-01-01', periods=big_data_count, freq='30s')
    big_data_cols = ['Name']
    for group in range(1, 31):
        big_data_cols.extend([f'date str {group}',
                              f'bool {group}',
                              f'int {group}',
                              f'float {group}',
                              f'str {group}'])
    big_data = []
    for i in range(0, big_data_count):
        row = [f'Name Item {(i + 1)}']
        for _ in range(0, 30):
            row.extend([str(dates[i]),
                        i % 2 == 0,
                        np.random.randint(10000, 100000),
                        10000 * np.random.random(),
                        chars[np.random.randint(0, 26)] * 15])
        big_data.append(row)
    df = pd.DataFrame(data=big_data, columns=big_data_cols)
    df.to_csv(csv_file, index=None)


if __name__ == '__main__':
    # 修改存放路径以及模拟数据量(默认 9 万)
    gen_big_data('./files/custom_big_data.csv', 180000)
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查看生成的数据格式，可以看到每一行有 151 列

保存数据后，先查看一下内存占用情况

import pandas as pd


def info_memory(csv_file: str):
    df = pd.read_csv(csv_file)
    print(df.info(memory_usage='deep'))


if __name__ == '__main__':
    info_memory('./files/custom_big_data.csv')
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打印结果如下，可以看到当前内存占用为 862.1MB

RangeIndex: 180000 entries, 0 to 179999
Columns: 151 entries, Name to str 30
dtypes: bool(30), float64(30), int64(30), object(61)
memory usage: 862.1 MB
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查看不同类型的内存占用情况

def info_memory_by_d_type(csv_file: str):
    df = pd.read_csv(csv_file)
    for d_type in ['bool', 'float64', 'int64', 'object']:
        d_type_selected = df.select_dtypes(include=[d_type])
        mem_mean_bit = d_type_selected.memory_usage(deep=True).mean()
        mem_mean_mb = mem_mean_bit / 1024 ** 2
        print(f'mean memory usage: {d_type:<7} - {mem_mean_mb:.3f} M')
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输出结果如下，其中 object 类型占用内存最多

mean memory usage: bool    - 0.166  M
mean memory usage: float64 - 1.329  M
mean memory usage: int64   - 1.329  M
mean memory usage: object  - 12.494 M
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二、优化方案

查看某个类型的内存占用量

def info_mem_usage_mb(pd_obj):
    if isinstance(pd_obj, pd.DataFrame):
        mem_usage = pd_obj.memory_usage(deep=True).sum()
    else:
        mem_usage = pd_obj.memory_usage(deep=True)
    # 转换为MB返回
    return f'{mem_usage / 1024 ** 2:02.3f} MB'
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int 和 float 类型

对于 int 和 float 类型的数据，Pandas 加载到内存中的数据，默认是 int64 和 float64。一般场景下的数据，用 int32 和 float32 就足够了，用 numpy.iinfo 和 numpy.finfo 可以打印对应类型的取值范围

Machine parameters for int32
---------------------------------------------------------------
min = -2147483648
max = 2147483647
---------------------------------------------------------------

Machine parameters for int64
---------------------------------------------------------------
min = -9223372036854775808
max = 9223372036854775807
---------------------------------------------------------------
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Machine parameters for float32
---------------------------------------------------------------
...
maxexp =    128   max =        3.4028235e+38
nexp =        8   min =        -max
---------------------------------------------------------------

Machine parameters for float64
---------------------------------------------------------------
...
maxexp =   1024   max =        1.7976931348623157e+308
nexp =       11   min =        -max
---------------------------------------------------------------
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分别优化 int 和 float 的类型

def optimize_int_and_float():
    df_int = df.select_dtypes(include=['int64'])
    df_int_converted = df_int.apply(pd.to_numeric, downcast='unsigned')
    df_float = df.select_dtypes(include=['float64'])
    df_float_converted = df_float.apply(pd.to_numeric, downcast='float')

    print('int before     ', info_mem_usage_mb(df_int))
    print('int converted  ', info_mem_usage_mb(df_int_converted))

    print('float before   ', info_mem_usage_mb(df_float))
    print('float converted', info_mem_usage_mb(df_float_converted))
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优化后的结果如下，内存减少 50% 左右

int before      41.199 MB
int converted   20.599 MB
float before    41.199 MB
float converted 20.599 MB
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object 类型中的普通 str 数据

获取 object 类型数据，并调用 describe() 展示统计信息

对于区分度较低的 str 1 到 str 30，一共只有 26 个可能的值，可以考虑转换为 Pandas 中的 categroy 类型，这里将区分度小于 40% 的列转换为 category 类型

def optimize_obj():
    df_obj = df.select_dtypes(include=['object'])
    df_obj_converted = pd.DataFrame()
    for col in df_obj.columns:
        unique_count = len(df_obj[col].unique())
        total_count = len(df_obj[col])
        # 将区分度小于40%的列转换为category类型
        if unique_count / total_count <= 0.4:
            df_obj_converted.loc[:, col] = df_obj[col].astype('category')
        else:
            df_obj_converted.loc[:, col] = df_obj[col]

    print('object before   ', info_mem_usage_mb(df_obj))
    print('object converted', info_mem_usage_mb(self.df_obj_converted))
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执行结果如下，降低了 300+M 的内存

object before    774.602 MB
object converted 409.047 MB
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object 类型中的 date 数据

def optimize_date_str():
    df_date = pd.DataFrame()
    df_date_converted = pd.DataFrame()
    for col_name in df.columns:
        if col_name.startswith('date str'):
            df_date.loc[:, col_name] = df[col_name]
            df_date_converted.loc[:, col_name] = pd.to_datetime(df[col_name])

    print('date before   ', info_mem_usage_mb(df_date))
    print('date converted', info_mem_usage_mb(df_date_converted))
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执行结果如下，也降低了 300+M 的内存

date before    391.388 MB
date converted 41.199 MB
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三、总体优化

综合以上的优化方法，并封装为类 PandasMemoryOptimizeDemo

import pandas as pd
import numpy as np


class PandasMemoryOptimizeDemo:
    df: pd.DataFrame
    df_int_converted: pd.DataFrame
    df_float_converted: pd.DataFrame
    df_obj_converted: pd.DataFrame
    df_date_converted: pd.DataFrame

    def __init__(self, csv_file: str):
        self.csv_file = csv_file
        self.df = pd.read_csv(self.csv_file)

    
    @staticmethod
    def info_mem_usage_mb(pd_obj):
        if isinstance(pd_obj, pd.DataFrame):
            mem_usage = pd_obj.memory_usage(deep=True).sum()
        else:
            mem_usage = pd_obj.memory_usage(deep=True)
        # 转换为 MB 返回
        return f'{mem_usage / 1024 ** 2:02.3f} MB'


        def optimize_int_and_float(self):
        df_int = self.df.select_dtypes(include=['int64'])
        self.df_int_converted = df_int.apply(pd.to_numeric, downcast='unsigned')
        df_float = self.df.select_dtypes(include=['float64'])
        self.df_float_converted = df_float.apply(pd.to_numeric, downcast='float')

        print('int before     ', self.info_mem_usage_mb(df_int))
        print('int converted  ', self.info_mem_usage_mb(self.df_int_converted))

        print('float before   ', self.info_mem_usage_mb(df_float))
        print('float converted', self.info_mem_usage_mb(self.df_float_converted))

    def optimize_obj(self):
        df_obj = self.df.select_dtypes(include=['object'])
        self.df_obj_converted = pd.DataFrame()
        for col in df_obj.columns:
            unique_count = len(df_obj[col].unique())
            total_count = len(df_obj[col])
            # 将区分度小于 40% 的列转换为 category 类型
            if unique_count / total_count <= 0.4:
                self.df_obj_converted.loc[:, col] = df_obj[col].astype('category')
            else:
                self.df_obj_converted.loc[:, col] = df_obj[col]

        print('object before   ', self.info_mem_usage_mb(df_obj))
        print('object converted', self.info_mem_usage_mb(self.df_obj_converted))

    def optimize_date_str(self):
        df_date = pd.DataFrame()
        self.df_date_converted = pd.DataFrame()
        for col_name in self.df.columns:
            if col_name.startswith('date str'):
                df_date.loc[:, col_name] = self.df[col_name]
                self.df_date_converted.loc[:, col_name] = pd.to_datetime(self.df[col_name])

        print('date before   ', self.info_mem_usage_mb(df_date))
        print('date converted', self.info_mem_usage_mb(self.df_date_converted))

    def optimize_all(self):
        self.optimize_int_and_float()
        self.optimize_obj()
        self.optimize_date_str()

        df_converted = self.df.copy()
        df_converted[self.df_int_converted.columns] = self.df_int_converted
        df_converted[self.df_float_converted.columns] = self.df_float_converted
        df_converted[self.df_obj_converted.columns] = self.df_obj_converted
        df_converted[self.df_date_converted.columns] = self.df_date_converted

        print('before   ', self.info_mem_usage_mb(self.df))
        print('converted', self.info_mem_usage_mb(df_converted))


if __name__ == '__main__':
    optimize_demo = PandasMemoryOptimizeDemo('./files/custom_big_data.csv')
    optimize_demo.optimize_all()
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执行结果如下，优化效果还是很明显的

before    862.149 MB
converted 105.207 MB
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四、直接优化 read_csv 方法

写代码的过程中，如果每次都按照这样的步骤，其实还是很繁琐，那能不能在调用 read_csv 方法时就进行优化呢？

接下来就一起来探索一下

在 PyCharm 中，点击 read_csv 进入源码，发现该方法提供了非常丰富的参数(50+)，这里只列举需要的参数

def read_csv(
    filepath_or_buffer: FilePath | ReadCsvBuffer[bytes] | ReadCsvBuffer[str],
    # General Parsing Configuration
    dtype: DtypeArg | None = None,
    converters=None,
    # Datetime Handling
    parse_dates=None,
    infer_datetime_format=False,
    keep_date_col=False,
    date_parser=None,
    low_memory=_c_parser_defaults["low_memory"],
    memory_map=False,
    storage_options: StorageOptions = None,
):
    # locals() should never be modified
    kwds = locals().copy()
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可以直接指定 dtype 和 parse_dates，最终代码如下

def big_data_optimized_read_csv(self):
    d_type_dict = {}
    date_indexes = []
    for i in range(1, 31):
        d_type_dict[f'int {i}'] = 'int32'
        d_type_dict[f'float {i}'] = 'float32'
        d_type_dict[f'str {i}'] = 'category'

        date_indexes.append(5 * (i - 1) + 1)
    self.df = pd.read_csv(self.csv_file, dtype=d_type_dict, parse_dates=date_indexes)
    print('optimized read_csv: ', self.info_mem_usage_mb(self.df))
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执行结果如下，内存占用也大大降低了

optimized read_csv:  105.207 MB