pyhomerun 0.4.0

Baseball statistics and MLB data for Python: sabermetrics calculations plus a zero-dependency client for the MLB Stats API.

⚾ pyhomerun

Baseball statistics and MLB data for Python — with zero dependencies.

pyhomerun does four things:

1. Sabermetrics — pure functions and stat-line dataclasses for batting, pitching, fielding, and team statistics (AVG, OBP, SLG, OPS, wOBA, wRC+, ERA, FIP, xFIP, Pythagorean win expectation, ...), plus situational run-expectancy (RE24). Plain numbers in, plain numbers out.
2. MLB data — MLBClient, a tiny client for the free, key-less MLB Stats API (players, season stats, teams, rosters, schedules, standings, boxscores, play-by-play), with optional disk caching, retry with backoff, and typo-tolerant player lookup.
3. CSV export — to_csv() turns a collection of stat lines into a CSV, no third-party spreadsheet library required.
4. A terminal command — pyhomerun standings, pyhomerun scores, pyhomerun player "...", pyhomerun export ... for a quick look without writing any Python.

It is built entirely on the Python standard library — installing it installs nothing else, and the test suite runs with stock Python.

Installation

Available on PyPI: https://pypi.org/project/pyhomerun/

pip install pyhomerun

Or straight from source:

git clone https://github.com/SRock44/pyhomerun
cd pyhomerun
pip install .

Requires Python 3.9+.

Quick start

Calculate statistics

import pyhomerun as bb

# Classic rate stats
avg = bb.batting_average(hits=200, at_bats=600)                     # 0.333
obp = bb.on_base_percentage(hits=200, walks=70, hit_by_pitch=5,
                            at_bats=600, sacrifice_flies=5)         # 0.404
tb  = bb.total_bases(hits=200, doubles=40, triples=5, home_runs=35) # 355
slg = bb.slugging_percentage(tb, at_bats=600)                       # 0.592
print(f"{avg:.3f}/{obp:.3f}/{slg:.3f}  OPS {bb.ops(obp, slg):.3f}")

# Advanced stats
bb.woba(walks=70, hit_by_pitch=5, singles=120, doubles=40, triples=5,
        home_runs=35, at_bats=600, sacrifice_flies=5)               # ~0.416
bb.fip(home_runs=18, walks=45, hit_by_pitch=5, strikeouts=190,
       innings_pitched=180)                                         # ~3.19
bb.era(earned_runs=3, innings_pitched=bb.innings(6.2))              # 4.05

Fetch MLB data

from pyhomerun import MLBClient

mlb = MLBClient()

# Find a player and pull their season batting line
judge = mlb.search_players("Aaron Judge")[0]
splits = mlb.player_stats(judge["id"], group="hitting", season=2025)
line = splits[0]["stat"]
print(judge["fullName"], line["avg"], line["homeRuns"], line["ops"])

# Today's games
for game in mlb.schedule():
    away, home = game["teams"]["away"], game["teams"]["home"]
    print(f'{away["team"]["name"]} at {home["team"]["name"]} — {game["status"]["detailedState"]}')

# Standings
for division in mlb.standings(season=2025):
    for record in division["teamRecords"]:
        print(record["team"]["name"], record["wins"], record["losses"])

Put them together: stat lines

BattingLine and PitchingLine bundle counting stats and expose every derived stat — and they build straight from API responses, so there's no field-mapping glue:

import pyhomerun as bb

mlb = bb.MLBClient()
player = mlb.search_players("Juan Soto")[0]
split = mlb.player_stats(player["id"], group="hitting", season=2025)[0]

line = bb.BattingLine.from_mlb(split)
print(player["fullName"], line.slash())      # 0.266/0.396/0.525
print(f"wOBA {line.woba():.3f}  wRC+ {line.wrc_plus():.0f}  BABIP {line.babip:.3f}")

Lines add together, so combining splits or seasons is just +:

career = bb.BattingLine()
for split in mlb.player_stats(player["id"], group="hitting", stat_type="yearByYear"):
    career += bb.BattingLine.from_mlb(split)
print(career.slash(), career.home_runs)

You can also build lines by hand — every field defaults to 0:

line = bb.BattingLine(at_bats=550, hits=150, doubles=30, triples=5, home_runs=25,
                      walks=70, hit_by_pitch=5, strikeouts=120, sacrifice_flies=5)
line.ops                       # 0.839...
line.wraa()                    # runs above average

arm = bb.PitchingLine(outs=540, hits=160, earned_runs=65, walks=50,
                      strikeouts=190, home_runs=18, hit_by_pitch=5)
arm.era, arm.whip, arm.fip()   # (3.25, 1.16..., 3.27...)

Team math

bb.pythagorean_expectation(800, 700)      # 0.566 expected win pct
bb.expected_wins(800, 700, games=162)     # 91.2 (Pythagenpat exponent)
bb.magic_number(leader_wins=90, second_place_losses=60)   # 13

Situational stats: run expectancy

from pyhomerun import BaseOutState, run_expectancy, run_value

# Runner on second, one out: how many runs does a team expect to score?
run_expectancy(BaseOutState(on_second=True, outs=1))       # 0.644

# A walk with the bases empty and nobody out — its run value:
before = BaseOutState()
after = BaseOutState(on_first=True)
run_value(before, after, runs_scored=0)                    # 0.37

CSV export

from pyhomerun import to_csv

roster_lines = {"Aaron Judge": line, "Juan Soto": other_line}  # name -> BattingLine
to_csv(roster_lines)                        # CSV text
with open("roster.csv", "w", newline="") as f:
    to_csv(roster_lines, file=f)

From the terminal

Installing the package also installs a pyhomerun command:

pyhomerun standings
pyhomerun scores 2025-10-01
pyhomerun player "Arron Judge"      # fuzzy: finds Aaron Judge despite the typo
pyhomerun teams
pyhomerun roster yankees
pyhomerun export hitting yankees --out yankees.csv

Responses are cached on disk for 5 minutes so re-running commands is instant and doesn't hammer the API. Same thing works as python -m pyhomerun ... if you'd rather not rely on the installed script being on PATH.

API reference

Every function has a full docstring with its formula and a worked example (help(bb.woba)).

Batting

Function	Statistic
batting_average(h, ab)	AVG
on_base_percentage(h, bb, hbp, ab, sf)	OBP
total_bases(h, 2b, 3b, hr)	TB
slugging_percentage(tb, ab)	SLG
ops(obp, slg)	OPS
ops_plus(obp, slg, lg_obp, lg_slg)	OPS+ (100 = league average)
isolated_power(slg, avg)	ISO
babip(h, hr, ab, k, sf)	BABIP
woba(...)	wOBA (customizable linear weights)
wraa(woba, pa)	wRAA (runs above average)
wrc(woba, pa) / wrc_plus(woba, park_factor)	wRC / wRC+ (100 = league average)
runs_created(h, bb, tb, ab)	Runs Created (Bill James)
plate_appearances(...)	PA
walk_rate(bb, pa) / strikeout_rate(k, pa)	BB% / K%
stolen_base_percentage(sb, cs)	SB%

Pitching

Function	Statistic
innings(6.2) / innings_from_outs(20)	Box-score notation → true innings
era(er, ip)	ERA
era_plus(era, lg_era) / era_minus(era, lg_era)	ERA+ / ERA- (100 = league average)
whip(bb, h, ip)	WHIP
fip(hr, bb, hbp, k, ip)	FIP (customizable constant)
xfip(fb, bb, hbp, k, ip)	xFIP (league HR/FB rate)
k_per_9 / bb_per_9 / hr_per_9 / h_per_9	Per-9 rates
k_bb_ratio(k, bb)	K/BB
left_on_base_percentage(...)	LOB%
game_score(...)	Bill James Game Score

Fielding

Function	Statistic
fielding_percentage(po, a, e)	FPCT
range_factor_per_game(po, a, g) / range_factor_per_9(po, a, inn)	RF/G, RF/9
caught_stealing_percentage(cs, sb)	CS%

Team

Function	Statistic
run_differential(rs, ra)	Run differential
pythagorean_expectation(rs, ra, exponent=2)	Pythagorean win %
pythagenpat_exponent(rs, ra, g)	Environment-aware exponent
expected_wins(rs, ra, g)	Expected win total (Pythagenpat)
magic_number(leader_wins, second_losses)	Clinch magic number

Situational

Function	Statistic
run_expectancy(BaseOutState(...))	RE24: expected rest-of-inning runs for a base-out state
run_value(before, after, runs_scored)	Run value of a play (change in expectancy + runs scored)
RE24_TABLE	The underlying {BaseOutState: float} matrix — swap in your own for exact per-season values

Stat lines

Class	What it does
BattingLine	Counting stats in; avg, obp, slg, ops, iso, babip, walk_rate, ... as properties, plus woba(), wraa(), wrc(), wrc_plus(), runs_created(), slash()
PitchingLine	Stores innings as outs for exact addition; era, whip, k_per_9, lob%, ... as properties, plus fip(), era_plus(), era_minus()
*.from_mlb(split)	Build either line directly from an MLBClient.player_stats() split
line + line	Combine splits/seasons field-by-field
to_csv(lines, file=None)	Export a mapping or iterable of lines to CSV (returns text, or writes to file)

MLB Stats API client

Method	Returns
MLBClient(timeout=10.0, cache_ttl=None, cache_dir=None, retries=0, backoff_factor=0.5)	— pass cache_ttl (seconds) to cache responses on disk, retries for transient-failure retry with backoff
.search_players(name)	Player matches (with MLBAM id)
.find_player(name)	Best-match player, tolerant of a typo in one name part (see docstring for what it can/can't fix)
.player(player_id)	Bio for one player
.player_stats(id, group, stat_type, season)	Stat splits ("hitting"/"pitching"/"fielding"; "season"/"career"/"yearByYear"/"gameLog")
.teams(season) / .roster(team_id)	Teams / active roster
.schedule(date, team_id)	Games for a date (default today)
.standings(season)	Division standings
.boxscore(game_pk) / .linescore(game_pk)	Game details
.play_by_play(game_pk)	Every play of a game, in order
.venues()	All MLB ballparks
.awards() / .award_recipients(award_id, season)	Awards / winners of one award
.draft(year)	Every pick of the amateur draft for a year
.get(path, **params)	Any other endpoint, as parsed JSON

All methods return plain dicts/lists parsed from the API's JSON — nothing is hidden, and .get() is an escape hatch to the API's many other endpoints. Errors raise pyhomerun.MLBAPIError.

Conventions

• Division by zero never raises: rate stats return 0.0 (or math.inf for ERA-style stats when runs scored without an out recorded). See each module's docstring.
• Innings must be true innings (6⅔, not the box-score 6.2) — convert with innings().
• League constants: woba and fip ship with representative modern-era defaults. For season-exact work, pass your own WobaWeights / FIP constant using values from the free FanGraphs Guts! page.
• Typed: the package ships a py.typed marker; all functions are annotated.

Running the tests

No test framework needed:

python -m unittest discover tests -v

(The suite also works under pytest if you prefer it.) The MLB client tests are fully offline — they never touch the network — and every docstring example runs as a doctest.

Roadmap

See ROADMAP.md for what's planned — a Monte Carlo season/playoff simulator, standings/schedule helpers, a sqlite3-backed local cache, and more, all still zero third-party dependencies. That constraint is the project's core mission, not a starting default.

Contributing

Contributions are welcome! See CONTRIBUTING.md. The short version: keep it dependency-free, add a docstring with formula + example to every public function, and include tests.

License and data

Code is MIT licensed. Data from the MLB Stats API is subject to the MLB copyright notice; this project is not affiliated with or endorsed by MLB.