diff --git a/.envrc b/.envrc
new file mode 100644
index 0000000..85618f1
--- /dev/null
+++ b/.envrc
@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+use flake
+
+# Add venv bin to PATH so development tools can be invoked more easily
+PATH_add .venv/bin
+
+# Add src to PYTHONPATH so Python and tools can find imports correctly
+path_add PYTHONPATH src
+
+# Add venv site packages to PYTHONPATH so imports work
+path_add PYTHONPATH ".venv/$NIX_PYTHON_SITE_PACKAGES"
+
+# Make pip use the venv (useful for ad-hoc temporary package installation)
+PIP_PREFIX="$(expand_path .)/.venv"
+export PIP_PREFIX
+export PIP_IGNORE_INSTALLED=1
+
+# These are annoying so stuff them all in one place
+PYTHONPYCACHEPREFIX="$(expand_path .)/.pycache"
+export PYTHONPYCACHEPREFIX
diff --git a/.gitignore b/.gitignore
index 8584735..92d0da7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,14 @@
-build
-__pycache__
-*.egg-info
+# Python artifacts
+/.coverage
+/.pycache
+/.pytest_cache
+/.venv
+/dist
+/build
+/*.egg-info
+
+# Direnv cache
+/.direnv
+
+# Nix artifacts
+/result
diff --git a/beangrow/compute_returns.py b/beangrow/compute_returns.py
index 7faabb9..bb4119d 100755
--- a/beangrow/compute_returns.py
+++ b/beangrow/compute_returns.py
@@ -87,7 +87,7 @@ def main():
prices.build_price_map(entries),
end_date,
output_reports,
- args.parallel, args.pdf)
+ parallel=args.parallel, pdf=args.pdf)
# Generate price reports.
output_prices = path.join(args.output, "prices")
diff --git a/beangrow/reports.py b/beangrow/reports.py
index 75dbe92..0fb4d7d 100755
--- a/beangrow/reports.py
+++ b/beangrow/reports.py
@@ -1,45 +1,42 @@
#!/usr/bin/env python3
-"""Calculate my true returns, including dividends and real costs.
+"""
+Calculate my true returns, including dividends and real costs.
"""
__copyright__ = "Copyright (C) 2020 Martin Blais"
__license__ = "GNU GPLv2"
-from os import path
-from typing import Any, Dict, List, Tuple, Optional
-from functools import partial
import collections
import datetime
import io
import logging
import multiprocessing
-import os
import platform
import subprocess
import tempfile
import typing
+from functools import partial
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple
-from dateutil.relativedelta import relativedelta
-import numpy as np
-
-import matplotlib.pyplot as plt
import matplotlib.dates as mdates
-import pandas
-from pandas.plotting import register_matplotlib_converters
-register_matplotlib_converters()
-
-from beancount.core import data
-from beancount.core import prices
+import matplotlib.pyplot as plt
+import numpy as np
+import pandas as pd
+from beancount.core import data, prices
from beancount.core.amount import Amount
from beancount.parser import printer
+from dateutil.relativedelta import relativedelta
+from matplotlib.axes import Axes
+from pandas.plotting import register_matplotlib_converters
-from beangrow.config_pb2 import Config, Group
-from beangrow.investments import AccountData
-from beangrow.investments import CashFlow
from beangrow import investments
from beangrow import returns as returnslib
+from beangrow.config_pb2 import Config, Group
+from beangrow.investments import AccountData, CashFlow
from beangrow.returns import Pricer
+register_matplotlib_converters()
# Basic type aliases.
Account = str
@@ -81,7 +78,8 @@
{title}
-
+