Skip opencv requirement if it's already installed in the env (i.e. via conda) (#616)

* Skip opencv requirement if it's already installed in the env

* pre-commit

* Check opencv conda version

* Check opencv conda version

setup.py

@@ -119,12 +119,21 @@ def parse_requirements(fname='requirements.txt', with_version=True):
     return packages
 
 
-# If first not installed install second package
-CHOOSE_INSTALL_REQUIRES = [('opencv-python-headless>=3', 'opencv-python>=3')]
-
 install_requires = parse_requirements()
-for main, secondary in CHOOSE_INSTALL_REQUIRES:
-    install_requires.append(choose_requirement(main, secondary))
+
+try:
+    # OpenCV installed via conda.
+    import cv2  # NOQA: F401
+    major, minor, *rest = cv2.__version__.split('.')
+    if int(major) < 3:
+        raise RuntimeError(
+            f'OpenCV >=3 is required but {cv2.__version__} is installed')
+except ImportError:
+    # If first not installed install second package
+    CHOOSE_INSTALL_REQUIRES = [('opencv-python-headless>=3',
+                                'opencv-python>=3')]
+    for main, secondary in CHOOSE_INSTALL_REQUIRES:
+        install_requires.append(choose_requirement(main, secondary))
 
 
 def get_extensions():