Bug: TypeError when using source: build for cutout (str / str path division)

Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of PyPSA-Eur.

• I have confirmed this bug exists on the current master branch of PyPSA-Eur.

Issue Description

Bug Description

When setting data.cutout.source: build in config.yaml, Snakemake throws a TypeError:

unsupported operand type(s) for /: 'str' and 'str'

Location

rules/build_electricity.smk, line 202

Cause

dataset_version() returns folder as a string (via .as_posix()),
but the code uses the / operator which requires a Path object:

cutout=CUTOUT_DATASET["folder"] / "{cutout}.nc" # ← Bug

In retrieve.smk the same issue is correctly handled with +:

CUTOUT_DATASET["folder"] + "/{cutout}.nc" # ← Correct

Fix

Change line 202 in rules/build_electricity.smk from:
cutout=CUTOUT_DATASET["folder"] / "{cutout}.nc"
to:
cutout=CUTOUT_DATASET["folder"] + "/{cutout}.nc"

Context

This only occurs when using historical weather years not available
on data.pypsa.org (e.g. 1965), which requires source: build.

Version

v2026.02.0

Reproducible Example

1. Set in config.yaml:
   data:
     cutout:
       source: build
       version: latest

2. Run:
   snakemake --cores 4 data/cutout/build/unknown/europe-1965-era5.nc

3. Error occurs immediately before any download starts

Expected Behavior

Snakemake should successfully create the build_cutout rule and
start downloading ERA5 data. The output path should be correctly
constructed as a string, consistent with how retrieve.smk handles
the same variable using + instead of /.

Installed Versions

PyPSA-Eur: v2026.02.0 OS: Ubuntu (Uni Server) / macOS (lokal) Python: 3.13

Read rules/build_electricity.smk around line 202 and compare its cutout path construction with the corresponding handling in rules/retrieve.smk. Run snakemake --cores 4 data/cutout/build/unknown/europe-1965-era5.nc and confirm the build_cutout rule is created and starts downloading ERA5 data without the TypeError.