algorithm - KeyError in Python Program but not in interpreter? -

here program trying implement prims algorithm minimum spanning tree. problem when execute program whole shows keyerror : 7 :

but when execute error causing portion in interpreter shows no error ?

my code :

edges = {             'a':{'d':1,'b':3},             'b':{'a':3,'c':4},             'c':{'b':4,'d':5,'a':2,'e':7},             'd':{'a':1,'c':5,'e':6},             'e':{'d':6,'c':7}         }  # here null represented -1 , infinity represented 1000  vertices = {             'a':[1000,-1],             'b':[1000,-1],             'c':[1000,-1],             'd':[1000,-1],             'e':[1000,-1]             }  def extractmin():     low = 'a'     key in vertices.keys():         if vertices[low][0]>vertices[key][0]:             low=key     del vertices[low]     return low  vertices['a'][0]=0  while not(vertices=={}):     u=extractmin()     print(str(u)+" ")     v in [x x in edges[u].keys()]:         if v in vertices , edges[u][v]<vertices[v][0]:             vertices[v][0]=edges[u][v] 

well, problem in function extractmin

first delete key a vertices because a minimum.

then, when calling again function extractmin try delete again (since vertices[low][0]>vertices[key][0] false) delete a , not exist in vertices.

you not putting in while loop becuase of condition v in vertices , know a not there