Dask DataFrames 入门

Dask is a parallel computing library for Python that extends NumPy and Pandas to datasets larger than RAM. Its dask.dataframe module provides a DataFrame API almost identical to Pandas, but instead of executing operations immediately, Dask builds a task graph and executes it lazily when you call .compute(). This allows Dask to parallelise work across multiple cores or even multiple machines with minimal code changes.

Dask is installed with pip install dask[dataframe]. The import convention is import dask.dataframe as dd. Under the hood, a Dask DataFrame is partitioned into many smaller Pandas DataFrames, each processed independently. Operations on the Dask DataFrame create a lazy task graph — nothing runs until .compute() is called. This separation between describing and executing computation is the key insight of Dask.

With Pandas, every operation executes immediately and eagerly. With Dask, operations return another Dask object representing the deferred computation. Only when you call .compute() does Dask actually read data and execute the task graph. This laziness allows Dask to optimise the plan before executing — for example, it can fuse consecutive filters to avoid loading data multiple times. Think of it like a recipe: Dask writes the recipe; .compute() cooks the meal.

A Dask DataFrame is split into partitions, each of which is a regular Pandas DataFrame. By default, dd.read_csv creates one partition per file (or one per 128 MB for large files). You can control this with blocksize. Checking ddf.npartitions shows how many partitions exist. More partitions enable more parallelism but add overhead; fewer partitions reduce overhead but limit parallelism. The sweet spot is typically a few hundred partitions.

Most common Pandas operations work identically in Dask: .head(), .tail(), .describe(), boolean indexing, .groupby(), .merge(), and .assign() all have Dask equivalents. The biggest difference is that you must call .compute() to materialise the result. Operations that Pandas handles in milliseconds may take seconds in Dask due to task graph overhead — so use Pandas for small data and Dask when data does not fit in RAM.

One of Dask's most useful features is reading multiple files at once using glob patterns. dd.read_csv('data/2024-*.csv') reads all matching files and creates one partition per file. This is perfect for data stored as monthly or daily partitioned files, a common pattern in data lakes. Dask aligns the schemas automatically, equivalent to manually looping and concatenating with Pandas but much simpler.

Before executing a complex Dask pipeline, you can inspect the task graph by calling result.visualize(), which generates a PNG diagram of all computation steps. This is valuable for understanding what Dask will execute and debugging unexpected slowness. The graph shows how partitions flow through filter, groupby, and aggregation steps, making it easy to spot redundant computations. Requires the graphviz package.

When you need to apply a custom Pandas function to a Dask DataFrame, use ddf.map_partitions(func). This applies func to each partition independently and returns a new Dask DataFrame. The function receives a regular Pandas DataFrame and must return one. This is the Dask equivalent of df.apply() and is how you integrate Dask with code that only understands Pandas.

Dask has multiple schedulers that control how tasks are executed. The 'synchronous' scheduler runs tasks sequentially in the current thread (useful for debugging). The 'threads' scheduler uses a thread pool (good for I/O-bound work). The 'processes' scheduler spawns multiple processes for CPU-bound work (bypasses Python's GIL). A Dask distributed cluster enables multi-machine execution. Specify the scheduler via compute(scheduler='threads').

It is common to process a large dataset with Dask and then bring the aggregated result into Pandas for the final analysis or visualisation. Use .compute() to convert a Dask DataFrame to Pandas. Going the other direction, dd.from_pandas(df, npartitions=4) converts a Pandas DataFrame to a Dask DataFrame, which is useful for testing Dask code on small data before scaling up to the full dataset.

Dask is not always the right tool. Use Pandas when your data fits in RAM (less than a few GB) — it is simpler and faster due to less overhead. Use Dask when data exceeds RAM but you want Pandas-like syntax and single-machine parallelism. Use SQL/database when data lives in a relational database and aggregations can be pushed to the database engine. Use Spark or BigQuery when you need multi-machine distributed processing at petabyte scale.

Test your understanding of Data Analysis concepts from this lesson.

In this lesson you learned: Dask DataFrames are lazily evaluated collections of Pandas partitions with a familiar API, .compute() triggers actual execution of the task graph, and map_partitions lets you apply any custom Pandas function across all partitions. Next up we look at Parquet format as a fast, columnar alternative to CSV for storing large datasets.