yield question with append

[gnomegordon]

when I try to use print the output the yield result, everything is shown as expected. The code and output is as below:

def triangles():
    oo = [1] 
    yield oo 
    oo.append(1)
    yield oo  
    oo.append(1)
    yield oo 
    oo.append(1)
    yield oo 

# Collect the first 3 rows of Triangle
n = 0
for t in triangles():
    print (t)
    n += 1
    if n == 3:  
        break

the output is:

Output:
[1]
[1, 1]
[1, 1, 1]

However when I tried to append the yield result further, like below

n = 0
results = []
for t in triangles():
    ## print (t)
    results.append(t)
    n += 1
    if n == 3:  
        break

# Print the results
for t in results:
    print(t)

the output becomes:

Output:
[1, 1, 1]
[1, 1, 1]
[1, 1, 1]

Actually it is hard for me to understand the result. why the 1st and 2nd row changes after further appending?

[deanhystad]

triangles returns the same list each time. Look at it this way:

def triangles():
    oo = [1]
    yield oo
    oo.append(1)
    yield oo
    oo.append(1)
    yield oo
    oo.append(1)
    yield oo


# Collect the first 3 rows of Triangle
saved_triangles = []
for t in triangles():
    saved_triangles.append(t)
    print(t, id(t))
print("\nSaved Triangles")
for t in saved_triangles:
    print(t, id(t))

Output:
[1] 2446984510720
[1, 1] 2446984510720      
[1, 1, 1] 2446984510720   
[1, 1, 1, 1] 2446984510720

Saved Triangles
[1, 1, 1, 1] 2446984510720
[1, 1, 1, 1] 2446984510720
[1, 1, 1, 1] 2446984510720
[1, 1, 1, 1] 2446984510720

As you can see from the output, the object returned by triangles() is always the same list (the id's match). The first time triangles() yields a list, the list contains [1]. The second yield returns the same list, but now the list contains [1, 1]. Printing the saved_triangles show that the first list yielded by triangles() magically morphed from containing [1] to containing [1, 1].

I think you want triangles to return a new list for each yield.

def triangles(count):
    for i in range(count):
        yield [1] * (i + 1)


# Collect the first 3 rows of Triangle
saved_triangles = []
for t in triangles(4):
    saved_triangles.append(t)
    print(t, id(t))
print("\nSaved Triangles")
for t in saved_triangles:
    print(t, id(t))

Output:
[1] 2280964861568
[1, 1] 2280964456128
[1, 1, 1] 2280964455616
[1, 1, 1, 1] 2280964886848

Saved Triangles
[1] 2280964861568
[1, 1] 2280964456128
[1, 1, 1] 2280964455616
[1, 1, 1, 1] 2280964886848

Now triangles yields a new list each time. Yielding a longer list doesn't change the length of previous yielded lists.

[gnomegordon]

Appreciate it, deanhystad: I understand it. the id helps.