petersburg

version number: 0.4.0 author: Will McGinnis

Overview

A framework for analyzing probabilistic decision processes as directed graphs with automatic sensitivity analysis.

Simulating and Predicting uncertain decisions

Petersburg is a framework based on the decision theoretic concept of an individual agent approaching a network of discrete decisions or probabilistic options. We model these networks as directed acyclic graphs that have a few extra concepts:

• Node Payoff: a potential reward for reaching some point
• Edge Cost: a cost of taking a certain path
• Edge Weight: a term related to the conditional likelihood of that edge being traversed (either a static number of classification model)

Using petersburg, you can:

• Build and simulate complex decision graphs
  • The most likely outcomes
  • The worst/best case scenarios
  • Distributions of outcomes through Monte Carlo simulation
• Model uncertainty with distribution-based node types
  • UniformNode: Outcomes uniformly distributed in a range
  • GaussianNode: Normally distributed outcomes
  • LogNormalNode: Log-normal distributions (always positive, heavy right tail)
  • PowerLawNode: Power law distributions for rare high-value events
• Automatic sensitivity analysis - identify which parameters impact outcomes the most
• Visualize decision graphs with Mermaid diagram export
• Predict outcomes using machine learning
  • FrequencyEstimator: Learn graph structure from historical data
  • MixedModeEstimator: Combine explicit structure with learned probabilities
• Real-world applications (see examples/case_studies/)
  • Drug development pipelines
  • Startup funding journeys
  • Product launches
  • Litigation strategy

Emergent Complexity: Start Simple, Grow Sophisticated

One of petersburg's core design principles is progressive refinement: you can start with a simplified model that captures the basic structure of a complex system, then gradually add realism as your understanding deepens or your data improves.

The Incremental Modeling Approach

Stage 1: Fixed Payoffs - Begin with deterministic outcomes to understand the structure

# Simple model: Fixed $5B blockbuster exit
{'payoff': 5000, 'after': [...]}

Stage 2: Distribution-Based Nodes - Add realistic uncertainty to outcomes

# More realistic: LogNormal distribution around $5B
{'type': 'lognormal', 'mu': 8.52, 'sigma': 0.4, 'after': [...]}

Stage 3: Learned Transitions - Use historical data to set edge probabilities

# Edge weights learned from data
from petersburg import FrequencyEstimator
estimator = FrequencyEstimator()
estimator.fit(X_features, y_outcomes)

Stage 4: Dynamic Edge Weights - Transitions depend on context/features

# Edge weights predicted by classifier based on features
{'node_id': 2, 'cost': 100, 'weight': trained_classifier}

Emergent Power Laws from Simple Compositions

A remarkable property of petersburg models is that simple binary transitions + continuous distributions = complex emergent behavior that matches real-world power laws.

For example, in our startup funding case study:

• Binary decisions at each stage (continue/exit/fail)
• LogNormal distributions at exit nodes
• Simple stage-by-stage filtering

This produces a portfolio outcome distribution that exhibits:

• Heavy right tails (rare mega-exits)
• Realistic failure rates (80%+ fail)
• Power law returns matching empirical VC data

You don't need to explicitly model power law complexity. By composing:

1. Sequential filtering (survival rates at each stage)
2. Multiplicative processes (ownership dilution)
3. Log-normal outcomes (valuation distributions)

...the framework naturally generates the complex emergent patterns we observe in real venture portfolios, pharmaceutical pipelines, and product launches.

When to Add Complexity

Use this decision framework:

Start simple. Add complexity only when:

• Sensitivity analysis shows a parameter matters
• You have data to support more sophisticated modeling
• Simpler models produce unrealistic outcomes

The goal is the simplest model that captures the essential dynamics of your decision problem.

Installation

Using pip

pip install petersburg

Optional extras:

pip install petersburg[visualization]  # networkx + matplotlib (to_networkx, plot)
pip install petersburg[graphviz]       # pygraphviz for Graph.plot() — also needs a system Graphviz
pip install petersburg[docs]           # sphinx toolchain for building docs/
pip install petersburg[examples]       # pandas on top of visualization (example scripts)
pip install petersburg[all]            # everything above plus dev tooling

From source (recommended for development)

git clone https://github.com/wdm0006/petersburg.git
cd petersburg
uv pip install -e ".[dev]"

Or using standard pip:

pip install -e ".[dev]"

Quick Start

Basic Graph Simulation

from petersburg import Graph

# Build a simple decision graph
spec = {
    0: {'payoff': 0, 'after': []},  # Start node — the one with an empty 'after' list
    1: {'payoff': 100, 'after': [{'node_id': 0, 'cost': 10, 'weight': 0.3}]},  # Success branch (30%)
    2: {'payoff': -50, 'after': [{'node_id': 0, 'cost': 5, 'weight': 0.7}]},   # Failure branch (70%)
    3: {'payoff': 0, 'after': [{'node_id': 1, 'cost': 0}, {'node_id': 2, 'cost': 0}]},  # Terminal node
}
g = Graph().from_dict(spec)

# Run simulation
outcomes = [g.get_outcome() for _ in range(10000)]
print(f"Expected value: ${sum(outcomes)/len(outcomes):.2f}")  # ≈ -$11.50 = 0.3·(+90) + 0.7·(−55)

How to read a graph dictionary:

• Keys are node IDs. A walk starts at the node whose after list is empty and ends at a node that nothing follows.
• A node's after list is its predecessors: 2: {'after': [{'node_id': 1, ...}]} draws an edge 1 → 2.
• At each node the walk picks among the edges leaving it with probability proportional to each edge's weight (normalized to sum to 1). A weight on a node's single outgoing edge has no effect — that edge is always taken. The 30/70 split in the example above therefore lives on the edges out of the start node (declared in nodes 1 and 2's after lists).

Reproducibility

Pass an integer seed to reproduce a simulation, or pass an existing NumPy Generator to control its random stream:

import numpy as np

seeded_graph = Graph(random_state=7).from_dict(spec)
generator_graph = Graph(random_state=np.random.default_rng(7)).from_dict(spec)

Graphs created with the same seed produce the same sequence of edge choices and stochastic payoffs. Omitting random_state keeps the default unseeded behavior.

Automatic Sensitivity Analysis

# Identify the most critical parameters
g.print_sensitivity_report(num_simulations=1000, perturbation=0.1, top_n=5)

# At most max_params parameters per type are analyzed (default 10). The report says
# how many of the model's parameters were covered; pass max_params=None for all.
g.print_sensitivity_report(num_simulations=1000, perturbation=0.1, top_n=5, max_params=None)

Visualizing the Graph

# Mermaid text export (no extra dependencies)
print(g.to_mermaid())

# Rendered image — needs the [graphviz] extra (pygraphviz) plus a system Graphviz
# install, and networkx/matplotlib from the [visualization] extra
g.plot("graph.png")

Distribution-Based Node Types

Model uncertainty with stochastic payoffs using different probability distributions:

from petersburg import Graph

# Create a graph with different distribution types
g = Graph()
g.from_dict({
    0: {'type': 'fixed', 'payoff': 0, 'after': []},  # Terminal node
    1: {
        'type': 'uniform',        # Uniform distribution
        'min_payoff': 50,
        'max_payoff': 150,
        'after': [{'node_id': 0, 'cost': 10}]
    },
    2: {
        'type': 'gaussian',       # Normal distribution
        'mean': 100,
        'std': 20,
        'after': [{'node_id': 0, 'cost': 10}]
    },
    3: {
        'type': 'lognormal',      # Log-normal (always positive)
        'mu': 4.5,
        'sigma': 0.5,
        'after': [{'node_id': 0, 'cost': 10}]
    },
    4: {
        'type': 'powerlaw',       # Power law (heavy tails)
        'scale': 50,
        'alpha': 2,
        'after': [{'node_id': 0, 'cost': 10}]
    },
    5: {
        'type': 'fixed',
        'payoff': 0,
        'after': [
            {'node_id': 1, 'cost': 0, 'weight': 0.25},
            {'node_id': 2, 'cost': 0, 'weight': 0.25},
            {'node_id': 3, 'cost': 0, 'weight': 0.25},
            {'node_id': 4, 'cost': 0, 'weight': 0.25},
        ]
    }
})

# Each simulation samples from the distributions
outcomes = [g.get_outcome() for _ in range(1000)]

Available Distribution Types:

• UniformNode: Payoffs uniformly distributed between min and max

  • Use case: Equal probability across a range (e.g., uncertain market size)
  • Parameters: min_payoff, max_payoff

• GaussianNode: Normally distributed payoffs

  • Use case: Natural variation around a mean (e.g., product sales)
  • Parameters: mean, std

• LogNormalNode: Log-normally distributed payoffs (always positive)

  • Use case: Multiplicative processes, skewed positive outcomes (e.g., startup valuations)
  • Parameters: mu, sigma (parameters of underlying normal distribution)

• PowerLawNode: Power law (Pareto) distributed payoffs

  • Use case: Heavy-tailed distributions with rare extreme events (e.g., viral content, breakthrough innovations)
  • Parameters: scale (minimum value), alpha (tail heaviness)

See examples/distribution_nodes_demo.py for detailed examples.

Comparing Initial Options

get_options() simulates the walk once per outgoing edge of the start node, so you can compare the initial choices by expected value:

options = g.get_options(iters=10_000)
# {1: 90.0, 2: -55.0}  # expected profit, keyed by each option's destination node id

Pass extended_stats=True for mean/max/min/count per option, or distribution=True for the full simulated outcome distribution of each option:

options = g.get_options(iters=10_000, distribution=True, alpha=0.05)
# each option carries mean/max/min/count plus the distribution keys:
# {
#     "std": 63.2,                       # sample standard deviation
#     "percentiles": {"p5": ..., "p25": ..., "p50": ..., "p75": ..., "p95": ...},
#     "p_loss": 0.184,                   # fraction of simulated outcomes < 0
#     "var_alpha": -18.4,                # alpha-quantile of outcomes (value at risk, outcome space)
#     "cvar_alpha": -31.7,               # mean of outcomes <= var_alpha (expected shortfall)
# }

alpha (default 0.05) is the quantile level for var_alpha/cvar_alpha; cvar_alpha <= var_alpha always holds, because expected shortfall averages the outcomes at or below the quantile. return_samples=True (only valid together with distribution=True) also returns each option's raw per-walk outcome samples as a numpy array. Calls without the new kwargs keep the historical return shape.

Error Handling

Every error petersburg raises on invalid use subclasses PetersburgError:

• ValidationError (also a ValueError) — invalid input values: adjacency matrices, sensitivity arguments, estimator targets and feature matrices, node payoffs, transition weights, and argument combinations such as get_options(return_samples=True) without distribution=True.
• SpecValidationError (a ValidationError that is also an AttributeError) — invalid graph specifications: unrecognized node types, references to unknown nodes, cycles, missing or multiple starting nodes, non-dict specifications.

from petersburg import PetersburgError

try:
    Graph().from_dict({1: {'payoff': 1, 'after': [{'node_id': 999}]}})
except PetersburgError as err:
    print(f"invalid graph: {err}")

Existing except ValueError and except AttributeError handlers keep working unchanged; new code should catch PetersburgError or ValidationError.

Serialization

Graph.to_dict() produces exactly the dictionary Graph.from_dict() consumes, so graphs with numeric weights round-trip losslessly:

reloaded = Graph().from_dict(g.to_dict())
assert reloaded.to_dict() == g.to_dict()

Serialization covers nodes (including per-type distribution payoff parameters), edge costs, and numeric transition weights. Edges weighted by estimator objects (trained classifiers) are deliberately not serializable — to_dict() raises ValidationError for them; rebuild such graphs from a numeric-weight specification and attach the classifiers afterwards.

Case Studies

See examples/case_studies/ for detailed real-world applications:

• Drug Development: Pharmaceutical R&D pipeline with Phase I-III trials
• Startup Funding: VC funding journey from pre-seed to exit
• Product Launch: New product introduction with market testing
• Litigation Strategy: Settlement vs. trial decision analysis

Each case study includes:

• Detailed markdown documentation with business context
• Python implementation with the petersburg framework
• Mermaid diagrams visualizing the decision flow
• Sensitivity analysis identifying critical parameters

Development

Running Tests

uv run pytest

Code Quality

# Format code
uv run black petersburg/ tests/ examples/

# Lint
uv run ruff check petersburg/ tests/ examples/

# Type check
uv run mypy petersburg/

Running Examples

The scripts in examples/ need pandas and matplotlib on top of the runtime dependencies. Install the examples extra first (it is also part of all):

uv pip install -e ".[examples]"

Several examples call plt.show(), which blocks under an interactive matplotlib backend, so set MPLBACKEND=Agg when running them unattended:

# Run a single example
MPLBACKEND=Agg python examples/necktie_paradox.py

# Run all examples
make examples

# Run all case studies
make case-studies

examples/print.py additionally needs a system Graphviz (plus pygraphviz) for its plot() call, and examples/stpetersburg_w_bankroll.py simulates 10 million games, so it takes several minutes.

Documentation

The Sphinx sources live in docs/. Build them locally with:

uv pip install -e ".[docs]"
sphinx-build -b html docs docs/_build

Contributing

Contributions are welcome! Please feel free to submit a Pull Request. For major changes, please open an issue first to discuss what you would like to change.

Example Static Graph

Here is a simple example of simulating the St. Petersburg Paradox game, with some slight variations. In this case the entrance fee is $10, and the game only has a maximum of 10,000 flips and is played 10,000,000 times.

from petersburg import Graph

if __name__ == '__main__':
    g = Graph()

    # st petersburg paradox w/ $10 entrance fee and only 10000 possible flips
    entrance_fee = 10
    gd = {1: {'payoff': 0, 'after': []}, 2: {'payoff': 0, 'after': [{'node_id': 1, 'cost': entrance_fee}]}}
    nn = 3
    for idx in range(10000):
        node_id = 2 * (idx + 1)
        payoff = 2 ** (idx + 1)
        gd[nn] = {'payoff': payoff, 'after': [{'node_id': node_id, 'cost': 0, 'weight': 1}]}
        nn += 1
        gd[nn] = {'payoff': 0, 'after': [{'node_id': node_id, 'cost': 0, 'weight': 1}]}
        nn += 1
    g.from_dict(gd)

    outcomes = []
    for _ in range(10000000):
        outcomes.append(g.get_outcome())

    print('\n\nSimulated Output')
    print(sum(outcomes))

Via simulation, the outcome of this is a profit of: $197,592,288. This will, of course, vary depending on the run, but will approach infinity as the number of games goes to infinity, regardless of cost-to-play.

Prediction (scikit-learn estimators)

There are two prediction objects, both scikit-learn style estimators that learn a decision graph from historical paths and predict outcomes through it:

• FrequencyEstimator — transition frequencies observed in the data
• MixedModeEstimator — frequencies plus per-transition classifiers (logistic regression by default) where enough training data exists

Both follow the scikit-learn contract:

import numpy as np
from petersburg import MixedModeEstimator
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import GridSearchCV

est = MixedModeEstimator(min_samples=100, clf=LogisticRegression())
est.fit(X, y)                  # y: one column per decision layer

est.classes_                   # fitted terminal labels
est.n_features_in_             # number of feature columns seen at fit

proba = est.predict_proba(X)   # shape (n_samples, n_classes); rows sum to 1
pred = est.predict(X)          # shape (n_samples,) — fitted terminal labels
est.score(X, y)                # terminal-layer accuracy

est.partial_fit(X_more, y_more)  # incremental update, both estimators

# hyperparameters live in __init__, so they are tunable:
search = GridSearchCV(
    est, {"min_samples": [10, 50, 100], "clf__C": [0.1, 1.0, 10.0]}, cv=3
)

Notes:

• predict() returns a one-dimensional label array (n_samples,) (it returned (n_samples, 1) before 0.2.0).
• Columns of predict_proba follow classes_; with a seeded random_state, the modal column always matches predict().
• FrequencyEstimator.fit(X=None, y) fits without features; n_features_in_ is absent in that case.
• Hyperparameters (min_samples, clf, clf_args) are stored verbatim per the scikit-learn parameter contract, so get_params(), set_params(), and clone() see exactly what was passed.

Full working examples in examples/estimation/.