Shapely MultiPolygon Construction will not accept entire set of Polygons

look at Creating Shapely MultiPolygons from shape file MultiPolygons and I detail below the process:

1) browse through the shapefile :

• with next()

import fiona
c = fiona.open('polygons.shp')
# the result is a Python iterator, so to read the first feature of the shapefile
print c.next()
# the result is a Python dictionary (GeoJSON)format
{'geometry': {'type': 'Polygon', 'coordinates': [[(249744.23153029341, 142798.16434689672), (250113.79108725351, 142132.95714436853), (250062.62130244367, 141973.76225829343), (249607.77877080048, 141757.71205576291), (249367.77424759799, 142304.68402918623), (249367.77424759799, 142304.68402918623), (249744.23153029341, 142798.16434689672)]]}, 'type': 'Feature', 'id': '0', 'properties': OrderedDict([(u'id', 1), (u'couleur', 1), (u'id_class', None)])}
# if you want the geometry only
print c.next()['geometry']
{'type': 'Polygon', 'coordinates': [[(249175.78991730965, 142292.53526406409), (249367.77424759799, 142304.68402918623), (249607.77877080048, 141757.71205576291), (249014.45396077307, 141876.13484290778), (249175.78991730965, 142292.53526406409)]]}
....

but as all the iterators, this method raises a built-in StopIteration exception at the end of the iterator or end of the file:

c.next()
Traceback (most recent call last): .....   
StopIteration

• so the solution is a list (iterator without the StopIteration exception)

c = fiona.open('polygons.shp')
features = list(c)
# and the result is a list of all the features in the shapefile:
# or
for features in fiona.open('polygons.shp'):
     ....

2) Therefore to convert the polygons to a multipolygon:

from shapely.geometry import MultiPolygo
multi = []
# append the geometries to the list
for pol in fiona.open('polygons.shp'):
    multi.append(shape(pol['geometry'])
# create the MultiPolygon from the list of Polygons
Multi = MultiPolygon(multi)
print Multi.wkt
'MULTIPOLYGON (((249744.2315302934148349 142798.1643468967231456, 250113.7910872535139788 142132.9571443685272243, ..., 249870.8182051893090829 142570.3083320840960369)))'

Or in one line with a list comprehension:

Multi = MultiPolygon([shape(pol['geometry']) for pol in fiona.open('polygons.shp')]

Note:

If there are different types of geometries in the shapefile (MultiPolygon embedded in polygons) use unary_union: "the unary union function can operate on different geometry types, not only polygons as is the case for the older cascaded unions"

from shapely.ops import unary_union
polygon1 = Polygon([(0, 0), (1, 1), (1, 0)])
polygon2 = Polygon([(1, 1), (2,2), (2, 0)])
poly  = MultiPolygon([polygon1,polygon2])
list = [polygon1,polygon2,poly]
result = unary_union(list)
print result
MULTIPOLYGON (((0.00 0.00, 1.00 1.00, 1.00 0.00, 0.00 0.00)), ((1.00 1.00, 2.00 2.00, 2.00 0.00, 1.0 1.00)))

So:

geoms =[shape(feature['geometry']) for feature in fiona.open("polygons.shp")]
result = unary_union(geoms) 

Thank to Marvin's answer. I had the same problem. So based on the hint, I rewrite the code to check if there is any case a shape(...) call lead to the creation of a MultiPolygon. Then this works perfectly.

polys = []
for pol in fiona.open(shapefile):
    poly = shape(pol['geometry'])
    if isinstance(poly, MultiPolygon):
        polys += poly
    else:
        polys.append(poly)
mp = MultiPolygon(polys)