Why is .append() not giving me the same answer as +=?

Question

The code challenge is to get all the courses in the values into a new list. This is the code that works for the challenge:
```
def courses(dictteachers):
  courselist = []
  for courses in dictteachers.values():
    courselist += courses
  return course_list
```
This is the code I initially wrote which failed:
```
def courses(dictteachers):
  courselist = []
  for courses in dictteachers.values():
    courselist.append(courses)
  return course_list
```
Why does course_list += courses work and course_list.append(courses) not?

Steven Parker · Accepted Answer

If you were to try printing out the returned lists in each case using the workspace, you'll notice:
+= extends the list with the contents of the new list ( [a, b, c, d] )
.append() extends the list by adding the new list as a single new item ( [a, b, [c, d]] )
This is discussed starting around 5:30 in the Lists (https://teamtreehouse.com/library/python-basics/python-data-types/lists) video of the Python Basics course.
If you wanted to, you could make .append() work with an additional loop:
py
def courses(dict_teachers):
  course_list = []
  for courses in dict_teachers.values():
    for each_course in courses:
      course_list.append(each_course)
  return course_list

But I think your first example is a better solution.

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Gary Gibson

Gary Gibson

Why is .append() not giving me the same answer as +=?

1 Answer

Steven Parker

Steven Parker