About x+=2 and x = x + 2

Question

I used to like the compactness of the expression, like using x += 2 instead of x = x + 2.
But, now I am learning Hamiltonian Path, or Graph Theory.
This is my graph dictionary:
g = {'a':['c'],
'b':['c','e','f'],
'c':['a','b','d','e'],
'd':['c'],
'e':['b','c','f'],
'f':['b','e']
}
According to the dict, each key is able to access to one of their element in the list.
So it form a graph
I like to find all paths which are from node 'a' to 'f'.
In the
class Graph:
I define a method call 'find_all_paths' which takes a start_vertex, end_vertex, and a path in the parentheses.
This is my method's code, and please pay attention to the third line which have += .

def find_all_paths(self, start_vertex, end_vertex, path = []):
graph = self.graph_dict
path += [start_vertex]
if start_vertex == end_vertex:
return [path]
if start_vertex not in graph:
return []
paths = []
for vertex in graph[start_vertex]:
if vertex not in path:
extended_path = self.find_all_paths(vertex, end_vertex, path)
for p in extended_path:
paths.append(p)
return paths

Using this to find all paths from 'a' to 'f', I get [['a', 'c', 'b', 'e', 'f', 'd']]

But now I test this but changing third line to path = path + [start_index] instead using +=:

I get[['a', 'c', 'b', 'e', 'f'], ['a', 'c', 'b', 'f'], ['a', 'c', 'e', 'b', 'f'], ['a', 'c', 'e', 'f']]
which is the expected answer.

What wrong have I done? Is there any explanation about why += cannot form the same output?

Answer 1

(3)
I doubt in a few point:
1) I am using Window XP, didn't renew since about 10 years ago, is it a reason?
2)I using Sololearn's Code Playground, Python, is there any difference with executing these code in other Python template?
3)As in 'path += [start_vertex]', I want the output starts with 'a' and ends with 'f', but the output '[['a', 'c', 'b', 'e', 'f', 'd']]' is even didn't ends with 'f', and the other code done it as well.

Answer 2

(1)
Not to waste your time, I put my code here. Let's check!
This is 'path = path + [start_vertex]' version:

class Graph:
def __init__ (self, graph_dict = None):
if graph_dict == None:
self.graph_dict = {}
else:
self.graph_dict = graph_dict

def find_all_paths(self, start_vertex, end_vertex, path = []):
graph = self.graph_dict
path = path + [start_vertex]
if start_vertex == end_vertex:
return [path]
if start_vertex not in graph:
return []
paths = []
for vertex in graph[start_vertex]:
if vertex not in path:
extended_path = self.find_all_paths(vertex, end_vertex, path)
for p in extended_path:
paths.append(p)
return paths

g = {'a':['c'],
'b':['c','e','f'],
'c':['a','b','d','e'],
'd':['c'],
'e':['b','c','f'],
'f':['b','e']
}

graph = Graph(g)
print (graph.find_all_paths('a','f'))

Output:
[['a', 'c', 'b', 'e', 'f'], ['a', 'c', 'b', 'f'], ['a', 'c', 'e', 'b', 'f'], ['a', 'c', 'e', 'f']]

Answer 3

(2)
This is 'path += [start_vertex]' version, I am just change that line, remain others the same:

class Graph:
def __init__ (self, graph_dict = None):
if graph_dict == None:
self.graph_dict = {}
else:
self.graph_dict = graph_dict

def find_all_paths(self, start_vertex, end_vertex, path = []):
graph = self.graph_dict
path += [start_vertex]
if start_vertex == end_vertex:
return [path]
if start_vertex not in graph:
return []
paths = []
for vertex in graph[start_vertex]:
if vertex not in path:
extended_path = self.find_all_paths(vertex, end_vertex, path)
for p in extended_path:
paths.append(p)
return paths

g = {'a':['c'],
'b':['c','e','f'],
'c':['a','b','d','e'],
'd':['c'],
'e':['b','c','f'],
'f':['b','e']
}

graph = Graph(g)
print (graph.find_all_paths('a','f'))

Output:
[['a', 'c', 'b', 'e', 'f', 'd']]

Answer 4

you lost me at x+=2 😦

Answer 5

You can try my code , I just bought a new laptop and tried on it , the problem remains...

Answer 6

This shouldn't be the case! += is just an alias for _=_+()