As of Shapely version 1.2.14, coordinates are slicable. This looks very similar to GEOSExtractLine, where a subset of the LineString can be extracted.
Here are some examples how you can slice coordinates to extract a new line object:
from shapely.geometry import LineString, Point
# Original LineString used for examples
line = LineString([(30, 50), (60, 100), (100, 70), (150, 120), (190, 80)])
line.length # 235.58873956203155
# Extract a LineString from first two coordinates
LineString(line.coords[:2]).length # 58.309518948453004
# .. and from the last two coordinates
LineString(line.coords[-2:]).length # 56.568542494923804
# Everything but the first and last coordinate
LineString(line.coords[1:-1]).length # 120.71067811865476
# Every second coordinate
LineString(line.coords[::2]).length # 163.35495027417937
# Reversed/flipped
LineString(line.coords[::-1]).length # 235.58873956203155
etc.
As for the general topic of getting GEOS functionality to Shapely, that is indeed a @sgillies answer. Generally, the functionality needs to also be supported in the GEOS C API.
To extract a new LineString using linear referencing methods, here is an add-on function in pure Python. If normalized=True, then uses normalized start/end linear referencing (i.e., within [0.0, 1.0]); otherwise the default is absolute linear referencing (i.e., within [0.0, total length]). Negative end is supported as the reverse distance from the end of the linestring.
def extract_line(line, start, end, normalized=False):
"""Extract LineString from `start` to `end`"""
assert start >= 0, '`start` must be greater than or equal to zero'
if end < 0:
if normalized:
end += 1.0
else:
end += line.length
assert end > start, '`end` must be greater than `start`'
start_c = line.interpolate(start, normalized).coords[0]
end_c = line.interpolate(end, normalized).coords[0]
for start_i, c in enumerate(line.coords):
if c == start_c:
start_c = None
break
if line.project(Point(c), normalized) >= start:
break
for end_i, c in enumerate(line.coords):
if c == end_c:
end_c = None
end_i += 1
break
if line.project(Point(c), normalized) >= end:
break
extracted = line.coords[start_i:end_i]
if start_c:
extracted.insert(0, start_c)
if end_c:
extracted.append(end_c)
return LineString(extracted)
Examples:
# Your example in the question:
extract_line(LineString([(0,0), (10,0)]), 0.5, 1, True).coords[:]
# [(5.0, 0.0), (10.0, 0.0)]
# My example, length of 10.0 removed from each end:
extract_line(line, 10, -10).coords[:]
# [(35.144957554275265, 58.57492925712544), (60.0, 100.0),
# (100.0, 70.0), (150.0, 120.0), (182.92893218813452, 87.071067811865476)]
Best Answer
You can check where Python is looking for modules using:
This will generally be a list of directories. If your Shapely module was not installed to one of these then Python wont be able to "see" it.
In this case you will either have to re-install it to one of the shown directories or make its location known to Python. You can do this in the code itself:
You will have to include the above within each Python file in which you import Shapely. For a more permanent solution you can set the PYTHONPATH environment variable with search paths you want to always include. For more information check out the sys module, particularly sys.path.