Parameter precision of transform_geom() not working for GeometryCollections

[SaCodematix]

I do get the error local variable 'new_coords' referenced before assignment when I call transform.transform_geom() with an indicated value for the optional precision parameter other than the default value of -1.

Expected behavior and actual behavior.

I expected to be able to transform a GeometryCollection with the precision of 3 decimal places through the optional parameter precision of transform.transform_geom().

Steps to reproduce the problem.

from fiona import transform

source_crs = "epsg:25832"
dest_src = "epsg:4326"
geom =  {'type': 'GeometryCollection', 'geometries': [{'type': 'LineString', 'coordinates': [(512381.8870945257, 5866313.311218272), (512371.23869999964, 5866322.282500001), (512364.6014999999, 5866328.260199999)]}]}

geom_transformed = transform.transform_geom(source_crs, dest_src, geom)
print("Transformed geom without optional parameter 'precision':", geom_transformed, "\n")

geom_transformed = transform.transform_geom(source_crs, dest_src, geom, precision=-1)
print("Transformed geom with optional default parameter 'precision=-1':", geom_transformed, "\n")

geom_transformed = transform.transform_geom(source_crs, dest_src, geom, precision=3)
print("Transformed geom with optional parameter 'precision=3':", geom_transformed)

Output:

# python3 transform_geom_test.py
Transformed geom without optional parameter 'precision': {'type': 'GeometryCollection', 'geometries': [{'type': 'LineString', 'coordinates': [(9.184271907836777, 52.94630492951154), (9.184113777710168, 52.94638582212857), (9.184015228795353, 52.9464397114269)]}]} 

Transformed geom with optional default parameter 'precision=-1': {'type': 'GeometryCollection', 'geometries': [{'type': 'LineString', 'coordinates': [(9.184271907836777, 52.94630492951154), (9.184113777710168, 52.94638582212857), (9.184015228795353, 52.9464397114269)]}]} 

Traceback (most recent call last):
  File "transform_geom_test.py", line 16, in <module>
    geom_transformed = transform.transform_geom(source_crs, dest_src, geom, precision=3)
  File "/home/sa/anaconda3/envs/wind/lib/python3.8/site-packages/fiona/transform.py", line 92, in transform_geom
    return _transform_geom(
  File "fiona/_transform.pyx", line 220, in fiona._transform._transform_geom
UnboundLocalError: local variable 'new_coords' referenced before assignment

Operating system

Linux Mint 19.3 (Tricia, Ubuntu Bionic)

Fiona and GDAL version and provenance

installed via conda:
Fiona 1.8.17
GDAL 3.1.3
Python 3.8.5

[rbuffat]

It looks like the precision parameter does not support "GeometryCollection": https://github.com/Toblerity/Fiona/blob/master/fiona/_transform.pyx#L166-L220

[SaCodematix]

You are right, @rbuffat, thanks for the hint of this code snippet!
I implemented the rounding myself now, but it would be nice to be able to use this optional precision parameter for GeometryCollections in future releases or at least have a more precise error message :)