MessyData

Synthetic dirty data generator. Define a schema in YAML, get a realistic messy DataFrame.

MessyData generates structured datasets from a declarative config and injects configurable anomalies — missing values, duplicates, invalid categories, bad dates, and outliers. Designed for testing data pipelines, validating data quality tooling, and feeding AI/ML workflows that need realistic imperfect data.

---

Claude Code Skill

MessyData includes a Claude Code skill that teaches any agent how to write configs, validate them, and use the CLI. Download SKILL.md and place it at:

~/.claude/skills/messydata/SKILL.md

Then invoke it with /messydata in any Claude Code session.

---

Install

uv pip install messydata
# or
pip install messydata

---

Quick Start

With a Claude Code agent (fastest)

With the skill installed, just describe what you need in plain English:

/messydata generate a retail transactions dataset starting from 2024-01-01, 500 rows
per day. Include product catalog, customer region, payment method, and a realistic
price distribution. Add some missing values across all columns, a few duplicate
records, and occasional outlier prices. Save it to retail.csv.

The agent will write the YAML config, validate it, and run the CLI to produce the file — no manual config writing needed.

---

CLI

# Generate to a file (format inferred from extension)
messydata generate my_config.yaml --rows 1000 --seed 42 --output data.csv
messydata generate my_config.yaml --rows 1000 --output data.parquet
messydata generate my_config.yaml --rows 1000 --output data.json

# Stream to stdout
messydata generate my_config.yaml --rows 1000

# Single day (requires temporal: true on a date field)
messydata generate my_config.yaml --start-date 2025-06-01 --rows 500

# Date range — --rows is rows per day
messydata generate my_config.yaml --start-date 2025-01-01 --end-date 2025-03-31 --rows 500 --output data.csv

# Validate a config without generating (exits 0/1 — useful in CI and agent loops)
messydata validate my_config.yaml

# Print the full JSON Schema for the config format
messydata schema

---

YAML + Python

# my_config.yaml
name: orders
primary_key: order_id

records_per_primary_key:
  type: lognormal
  mu: 2.0
  sigma: 0.5

anomalies:
  - name: missing_values
    prob: 1.0   # always inject
    rate: 0.05  # 5% of cells set to NaN

fields:
  - name: order_id
    dtype: int32
    unique_per_id: true
    nullable: false
    distribution:
      type: sequential
      start: 1

  - name: order_date
    dtype: object
    unique_per_id: true
    nullable: false
    temporal: true                  # marks this as the date anchor
    distribution:
      type: sequential
      start: "2024-01-01"

  - name: amount
    dtype: float32
    nullable: false
    distribution:
      type: lognormal
      mu: 3.5
      sigma: 0.75

  - name: status
    dtype: object
    nullable: false
    distribution:
      type: weighted_choice
      values: [pending, shipped, delivered, cancelled]
      weights: [0.1, 0.3, 0.5, 0.1]

from messydata import Pipeline

pipeline = Pipeline.from_config("my_config.yaml")

# All rows, sequential dates
df = pipeline.run(n_rows=1000, seed=42)

# All rows pinned to a single date
df = pipeline.run_for_date("2025-06-01", n_rows=500)

# One generation pass per day, concatenated
df = pipeline.run_date_range("2025-01-01", "2025-03-31", rows_per_day=500)

Python-first

All distribution and anomaly types are importable as Python classes with full IDE support:

from messydata import (
    DatasetSchema, Pipeline,
    FieldSpec, AnomalySpec,
    Lognormal, WeightedChoice, Sequential,
)

schema = DatasetSchema(
    name="orders",
    primary_key="order_id",
    records_per_primary_key=Lognormal(mu=2.0, sigma=0.5),
    fields=[
        FieldSpec(name="order_id", dtype="int32",
                  distribution=Sequential(start=1),
                  unique_per_id=True, nullable=False),
        FieldSpec(name="amount", dtype="float32",
                  distribution=Lognormal(mu=3.5, sigma=0.75),
                  nullable=False),
        FieldSpec(name="status", dtype="object",
                  distribution=WeightedChoice(
                      values=["pending", "shipped", "delivered"],
                      weights=[0.2, 0.5, 0.3])),
    ],
    anomalies=[AnomalySpec(name="missing_values", prob=1.0, rate=0.05)],
)

df = Pipeline(schema).run(n_rows=1000, seed=42)

---

YAML Config Reference

Top-level keys

Key	Type	Required	Description
name	string	yes	Dataset identifier
primary_key	string	no (default: id)	Field used as the primary grouping key
records_per_primary_key	distribution block	yes	How many rows to generate per primary key value — accepts any continuous distribution
fields	list of field specs	yes	Column definitions
anomalies	list of anomaly specs	no	Data quality issues to inject

Row count: run(n_rows=N) generates approximately N rows. Because each primary key group is sampled from records_per_primary_key, the actual count may differ slightly. Each group always has at least 1 row.

---

Field spec properties

Property	Type	Required	Default	Description
name	string	yes	—	Column name in the output DataFrame
dtype	string	no	object	Pandas dtype: int32, int64, float32, float64, object, bool
distribution	distribution block	yes	—	How values are sampled (see Distribution Reference)
unique_per_id	bool	no	false	If true, one value is drawn per primary key group and repeated for all rows in that group
nullable	bool	no	true	Marks the field as nullable — used by anomaly injection
temporal	bool	no	false	Marks this field as the date anchor for run_for_date / run_date_range. Exactly one field per schema.

unique_per_id: true is appropriate for entity-level attributes that don't vary per transaction — e.g., a customer's region, a store's tier, a payment method for an order.

---

Distribution reference

Each distribution block requires a type key. All other keys are parameters for that distribution type.

Continuous distributions

type	Parameters	Notes
uniform	min, max	Uniform over [min, max]
normal	mean, std	Gaussian
lognormal	mu, sigma	Log-normal — good default for prices, quantities, durations
weibull	a, scale (default 1.0)	Parametrised by shape a
exponential	scale (default 1.0)	Rate = 1 / scale
beta	a, b	Output in [0, 1] — useful for rates and probabilities
gamma	shape, scale (default 1.0)	General-purpose skewed positive
mixture	components, weights	Weighted blend of continuous distributions — see below

Categorical distributions

type	Parameters	Notes
weighted_choice	values, weights	Draws from a fixed list. weights must sum to 1.
weighted_choice_mapping	columns, weights	Draws correlated multi-column outcomes from a joint table — see below

Special distributions

type	Parameters	Notes
sequential	start, step (default 1)	Auto-incrementing. start can be an integer or a date string ("2023-01-01"). Each primary key group advances by step.

---

weighted_choice — categorical with probabilities

distribution:
  type: weighted_choice
  values: [north, south, east, west]
  weights: [0.4, 0.3, 0.2, 0.1]

---

weighted_choice_mapping — correlated multi-column categorical

When two or more columns are always correlated (e.g., product_id and product_name always appear together), use a single weighted_choice_mapping field. All lists under columns must have the same length — each index is one joint outcome.

- name: product        # field name is a placeholder; actual columns come from `columns:`
  dtype: object
  distribution:
    type: weighted_choice_mapping
    columns:
      product_id:   [1001,        1002,    1003,      1004,         1005]
      product_name: [Widget,      Gadget,  Doohickey, Thingamajig,  Whatsit]
    weights: [0.4, 0.2, 0.2, 0.1, 0.1]

This adds product_id and product_name as separate columns — guaranteed consistent. The placeholder name: product is not added to the DataFrame.

---

sequential — auto-incrementing integers or dates

# Integer sequence starting at 1
distribution:
  type: sequential
  start: 1
  step: 1

# Date sequence — start must be a YYYY-MM-DD string
distribution:
  type: sequential
  start: "2023-01-01"
  step: 1           # advances by 1 day per primary key group

---

mixture — weighted blend of continuous distributions

# Bimodal price distribution: budget items + premium items
distribution:
  type: mixture
  components:
    - type: normal
      mean: 15.0
      std: 3.0
    - type: lognormal
      mu: 5.0
      sigma: 0.8
  weights: [0.6, 0.4]

mixture only supports continuous component types (uniform, normal, lognormal, weibull, exponential, beta, gamma). Categorical and sequential types cannot be used as components.

---

Anomaly reference

Each anomaly has two required fields:

Field	Type	Description
prob	float [0–1]	Probability this anomaly fires on a given run. 1.0 = always inject.
rate	float [0–1]	Fraction of eligible rows or cells affected when the anomaly fires.

Example: prob: 0.3, rate: 0.05 means a 30% chance the anomaly is active; when active, 5% of eligible rows are affected. Use prob: 1.0 for deterministic injection.

Anomaly types

name	columns	Extra params	Description
missing_values	any or list	—	Sets values to NaN. any targets all columns.
duplicate_values	—	—	Duplicates a fraction of rows and appends them.
invalid_category	list	—	Replaces values in the listed columns with "INVALID".
invalid_date	list	—	Replaces values in the listed columns with "9999-99-99".
outliers	list	distribution	Replaces values with samples from the specified distribution.

anomalies:
  - name: missing_values
    prob: 1.0
    rate: 0.08
    columns: any

  - name: duplicate_values
    prob: 0.5
    rate: 0.03

  - name: invalid_category
    prob: 0.3
    rate: 0.05
    columns: [product_name, region]

  - name: invalid_date
    prob: 0.4
    rate: 0.02
    columns: [order_date]

  - name: outliers
    prob: 0.2
    rate: 0.05
    columns: [unit_price]
    distribution:
      type: lognormal
      mu: 6.0
      sigma: 0.5

columns: any is a special string value (not a YAML list). It is accepted by missing_values and tells the injector to target all columns. All other anomaly types require an explicit column list.

---

Simulating Live & Historical Data

Mark one date field as temporal: true to unlock date-aware generation modes.

- name: transaction_date
  dtype: object
  unique_per_id: true
  nullable: false
  temporal: true                    # ← enables date-aware modes
  distribution:
    type: sequential
    start: "2024-01-01"

Then use run_for_date or run_date_range instead of run:

from datetime import date
from messydata import Pipeline

pipeline = Pipeline.from_config("config.yaml")

# Generate for a single day
df = pipeline.run_for_date("2025-06-01", n_rows=500)

# Generate a historical range
df = pipeline.run_date_range("2025-01-01", "2025-03-31", rows_per_day=500)

# Hybrid: backfill to today, then run daily from cron
df = pipeline.run_date_range("2025-01-01", date.today(), rows_per_day=500)

Or from the CLI:

# Single day
messydata generate config.yaml --start-date 2025-06-01 --rows 500

# Date range (--rows = rows per day)
messydata generate config.yaml --start-date 2025-01-01 --end-date 2025-03-31 --rows 500

Each day is generated independently with its own seed offset — anomaly patterns vary across days. Anomalies that target the date field (e.g. invalid_date) still apply, so filter them out if you need clean date values downstream.

---

Full Example Config

# examples/retail_config.yaml
name: retail
primary_key: id

# Average ~33 rows per transaction group (exp(3.5) ≈ 33)
records_per_primary_key:
  type: lognormal
  mu: 3.5
  sigma: 0.75

anomalies:
  - name: missing_values
    prob: 1.0
    rate: 0.05
    columns: any

  - name: duplicate_values
    prob: 0.3
    rate: 0.02

  - name: invalid_category
    prob: 0.2
    rate: 0.03
    columns: [product_name, payment_method]

  - name: invalid_date
    prob: 0.2
    rate: 0.02
    columns: [date]

  - name: outliers
    prob: 0.2
    rate: 0.05
    columns: [unit_price]
    distribution:
      type: lognormal
      mu: 6.0
      sigma: 0.5

fields:
  # Transaction ID — sequential integer, one per primary key group
  - name: id
    unique_per_id: true
    dtype: int32
    nullable: false
    distribution:
      type: sequential
      start: 1

  # Transaction date — one date per group, advancing daily
  - name: date
    unique_per_id: true
    dtype: object
    nullable: false
    temporal: true
    distribution:
      type: sequential
      start: "2023-01-01"
      step: 1

  # Store — entity attribute, fixed per transaction
  - name: store_id
    unique_per_id: true
    dtype: int32
    nullable: false
    distribution:
      type: weighted_choice
      values: [1, 2, 3, 4, 5]
      weights: [0.5, 0.2, 0.1, 0.1, 0.1]

  # Customer — entity attribute
  - name: customer_id
    unique_per_id: true
    dtype: int32
    nullable: false
    distribution:
      type: uniform
      min: 1000
      max: 9999

  # Product — correlated ID + name from a fixed catalog
  - name: product
    unique_per_id: false
    dtype: object
    nullable: false
    distribution:
      type: weighted_choice_mapping
      columns:
        product_id:   [1001, 1002, 1003, 1004, 1005]
        product_name: [A,    B,    C,    D,    E]
      weights: [0.4, 0.2, 0.2, 0.1, 0.1]

  # Quantity — uniform integer per line item
  - name: quantity
    unique_per_id: false
    dtype: int32
    nullable: false
    distribution:
      type: uniform
      min: 1
      max: 10

  # Unit price — log-normal, typical for retail prices
  - name: unit_price
    unique_per_id: false
    dtype: float32
    nullable: false
    distribution:
      type: lognormal
      mu: 3.5
      sigma: 0.75

  # Payment method — entity attribute for the transaction
  - name: payment_method
    unique_per_id: true
    dtype: object
    nullable: false
    distribution:
      type: weighted_choice
      values: [credit_card, cash, store_credit]
      weights: [0.8, 0.15, 0.05]

---

Working with AI Agents

MessyData's YAML format is designed to be written by language models without any procedural code. The config is declarative, self-describing, and maps directly to real-world data concepts.

Why it works well for agents

• Small fixed vocabulary — 11 distribution types with 1–3 parameters each; an agent can enumerate them all from this README
• Domain-transparent — field names, distribution types, and anomaly names use standard data engineering language
• Composable — anomalies are independent specs; an agent can add, remove, or tune one without touching the rest of the config
• No procedural logic — the agent describes the schema, not the generation procedure

Prompt template

Generate a MessyData YAML config for a [domain] dataset.

Dataset requirements:
- Primary entity: [e.g., customer_id, order_id]
- Fields: [describe each field — name, expected distribution, whether it varies per row or per entity]
- Target ~[N] rows per primary key group on average
- Anomalies to inject: [list types and approximate rates]

Distribution types available:
  Continuous: uniform, normal, lognormal, weibull, exponential, beta, gamma, mixture
  Categorical: weighted_choice, weighted_choice_mapping
  Special: sequential

Rules:
- Use lognormal for prices, durations, and revenue
- Use weighted_choice for any field with a fixed set of categories
- Use weighted_choice_mapping when two columns are always correlated (e.g. product_id + product_name)
- Set unique_per_id: true for entity attributes that don't vary per row within a group
- Use prob: 1.0 on anomalies that should always be present; lower values for probabilistic injection
- Keep rate below 0.3 — above that, data becomes mostly noise

Patterns to follow

Do	Avoid
Use lognormal for prices, durations, and counts	Using uniform for everything
Use weighted_choice_mapping for correlated column pairs	Separate weighted_choice fields that can produce inconsistent pairs
Set unique_per_id: true on entity-level attributes	Per-row variation on fields that belong to the entity
Use prob < 1.0 for realistic non-determinism	prob: 1.0, rate: 1.0 — destroys the dataset
Target specific columns on category/date anomalies	columns: any on anomalies that should only touch specific fields
Use mixture for bimodal distributions	Using a single distribution when the real data has two regimes

---

Output

Pipeline.run() returns a pandas.DataFrame.

• Column names and dtypes match the field specs
• Row count is approximately n_rows — may vary slightly due to the records_per_primary_key distribution
• The seed parameter makes generation fully reproducible
• Anomaly injection happens in-place; no indicator columns are added

df = Pipeline.from_config("my_config.yaml").run(n_rows=1000, seed=42)

df.info()               # column names, dtypes, non-null counts
df.isna().sum()         # verify injected nulls
df.duplicated().sum()   # verify injected duplicates
df.describe()           # distribution summary