从 Pandas 转向 Polars：新手常见的 10 个问题与优化建议

Polars 速度快、语法现代、表达力强，但很多人刚上手就把它当 Pandas 用，结果性能优势全都浪费了。

下面是新手最容易犯的 10 个错误，以及对应的解决思路。

1、直接 read_csv 而不用 scan_*

新手拿到一个大 CSV，上来就这么写：

 df=pl.read_csv("events.csv")

这会把整个文件一口气塞进内存。文件一旦上了 GB 级别，内存直接爆掉，性能也跟着完蛋。正确做法是用惰性扫描：

 lf=pl.scan_csv("events.csv")

所有操作保持惰性状态，直到最后调用

.collect()

。

这样做的好处是优化器可以把过滤和投影操作下推到扫描阶段，I/O 和内存占用都会大幅下降。

2、还在用 Python 循环或 .apply()

想给数据加个新列，很多人会写成这样：

 df=df.with_columns(       pl.col("price").apply(lambdax: x*1.19)   )

这种写法强迫 Python 逐行处理，完全没有向量化可言，慢得离谱。换成原生表达式：

 df=df.with_columns(       (pl.col("price") *1.19).alias("price_with_vat")   )

这样操作会跑在 Rust 层面，有 SIMD 加速，还能融合进查询计划里。性能差距就变得很大了

3、collect() 调用太早、太频繁

新手经常写出这种流水线：

 df1=lf.filter(...).collect()   df2=df1.with_columns(...).collect()

每调一次

.collect()

，整个数据集就要完整物化一遍。应该把所有操作串起来，最后只 collect 一次：

 result= (       lf.filter(...)         .with_columns(...)         .groupby(...)         .agg(...)   )    df=result.collect()

单次

.collect()

让优化器有机会做全局优化，计算量能省下一大截。

4、不做列裁剪（投影下推）

比如加载了一张 200 多列的宽表，实际只用到 4 列——但整张表还是全读进来了。正确做法是是尽早筛选列：

 lf=lf.select(["user_id", "country", "revenue", "event_time"])

Polars 会把投影下推到扫描层，从磁盘上读取时只读这几列。配合 Parquet 格式效果更明显，速度提升非常可观。

5、太早转成 Pandas

有人习惯这么干：

 pd_df=lf.collect().to_pandas()

还没过滤、没分组、没聚合，就先转成 Pandas 了，结果几千万行数据全在 Pandas 里慢慢磨。合理的做法是先在 Polars 里把重活干完：

 cleaned=lf.filter(...).groupby(...).agg(...)   pdf=cleaned.collect().to_pandas()

Polars 是计算引擎，Pandas 只是展示层，搞反了性能优势就没有了。

6、搞混 DataFrame、LazyFrame 和 Expr

新手容易写出这种代码：

 lf.groupby("user_id").sum()  

或者：

 df.with_columns(lf.col("price"))

原因是没搞清楚三种核心类型的区别。

要记住：DataFrame 是已经物化的数据；LazyFrame 是查询计划；Expr 是列表达式。

 lf=pl.scan_csv("file.csv")   # LazyFrame   df=lf.collect()              # DataFrame   expr=pl.col("amount")        # Expr

模型清晰了，才能避开各种隐蔽 bug 也才能让优化器真正发挥作用。

7、以为 .unique()和 Pandas 一样

有些人期望

.unique()

返回排序后的结果，但 Polars 默认保留原始顺序：

 lf.select(pl.col("country").unique())

这跟 Pandas 的行为是不一样，所以很容易出逻辑错误。如果需要排序就显式加上：

 lf.select(pl.col("country").unique().sort())

显式排序能避免跨框架时的隐性差异。

8、不管数据类型

CSV 里的数据经常乱七八糟：

"19.99", "20", "error", ""

Pandas 碰到这种情况会默默建个 object 列，而 Polars 会尝试推断类型，但新手往往不验证。

这时在扫描时直接指定类型更靠谱：

 lf=pl.scan_csv(       "orders.csv",       dtypes={"price": pl.Float64}   )

或者读完再转：

 df=df.with_columns(pl.col("price").cast(pl.Float64))

类型明确的管道更稳定、更可预测，跑起来也更快。

9、大数据聚合不开流式模式

几十亿行数据做 groupby：

 lf.groupby("user_id").agg(...)

内存肯定撑不住，程序就直接崩掉了。这时要开启流式模式：

 result= (       lf.groupby("user_id")         .agg(pl.col("amount").sum())         .collect(streaming=True)   )

流式处理会分块执行特别适合 ETL 场景和日志分析管道。

10、多次 with_columns 而不是合并表达式

新手容易这么写：

 df=df.with_columns(pl.col("a") +pl.col("b"))   df=df.with_columns(pl.col("c") -pl.col("d"))   df=df.with_columns(pl.col("e") *1.19)

三次调用，三个独立步骤，没法融合优化。可以将他们合并到一个表达式块里：

 df=df.with_columns([       (pl.col("a") +pl.col("b")).alias("ab"),       (pl.col("c") -pl.col("d")).alias("cd"),       (pl.col("e") *1.19).alias("e_vat")   ])

Polars 会把这些表达式融合成一个优化后的操作。步骤少了自然就快了。

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总结

从 Pandas 转过来的人，很容易带着旧习惯写 Polars 代码，结果性能优势全没了。上面这些点总结下来就是：惰性优先、表达式为主、最后才 collect、别用 Python 循环、列要有明确类型、多用 LazyFrame、善用投影下推和谓词下推、大数据开流式处理。