[Bug] KeyError highway crashes plot() when Overpass returns partial/empty data

Bug: KeyError: 'highway' crashes plot() when Overpass returns partial/empty data

Describe the bug

prettymaps.plot() crashes with KeyError: 'highway' when the Overpass API returns a partial or empty response (throttling, rate-limit, network timeout). This is the primary blocker that prevents rendering maps for many cities.

To reproduce

import prettymaps
prettymaps.plot("Paris, France", radius=1200)
# → Fetching geodataframes took 123.25 seconds
# → KeyError: 'highway'

Reproduced on 2026-08-19 with overpass-api.de under throttling. Zurich also fails at radius 900, 1200, and 1440.

Expected behavior

The function should either skip the empty layer and render the remaining data, or raise a meaningful error explaining that the OSM fetch returned no data for a required layer.

Root cause

fetch.py — unified_osm_request (~line 672): When iterating layers, the code indexes all_features[key] without checking the column exists:

filtered_features = all_features[~pd.isna(all_features[key])]   # KeyError if 'highway' column absent

The bare except Exception as e: catches this and returns GeoDataFrame(geometry=[]) — an empty GDF with no columns at all.

draw.py — graph_to_shapely (line 201): Then unconditionally accesses the missing column:

gdf["width"] = gdf["highway"].map(highway_to_width)   # KeyError: 'highway'

Suggested fix

draw.py — guard at top of graph_to_shapely:

if len(gdf) == 0 or "highway" not in gdf.columns:
    return MultiLineString([])   # empty geometry → layer silently skipped

fetch.py — in the except branch, preserve schema so downstream code doesn't crash:

empty = GeoDataFrame(geometry=[])
layer_tags = layers_dict.get(layer, {}).get("tags", {})
for key in layer_tags.keys():
    if key not in empty.columns:
        empty[key] = None
gdfs[layer] = empty

Environment

• prettymaps v1.4.2
• Python 3.10.18 (Homebrew)
• macOS 15, ARM64
• osmnx 1.2.2, geopandas, matplotlib (Agg backend)