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:

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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

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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:

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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.

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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.