Notes for Session 04¶
Instructor: Chris
4/19/2018
A collection of notes to go over in class, to keep things organized.
Issues that came up during the week.¶
Mutable default parameters¶
There was a video on this – any questions about it?
If not then we’ll move on…
This is a real “gotcha” in Python. One of you wrote a non-recursive solution to the sum_series problem. It worked great – EXCEPT if it got called more than once! Any idea what the problem is?
def sum_series(nth=1, sequence=[0,1]):
"""
Generate a list of sums given a seed and return the Nth number.
"""
for i in range(2, nth):
sequence.append(sequence[i-2] + sequence[i-1])
return sequence[nth-1]
So this uses the logic of starting out with the first two values in the series, and then looping to build up the series from there.
And [0, 1] is set as a default to start the series off – the start of the Fibonacci series. So if you pass in only one argument, you should get the Fibonacci number:
Remember that the start of the Fibonacci series is:
0, 1, 1, 2, 3, 5, 8, 13, ...
What happens when you run this code:
In [21]: sum_series(5)
Out[21]: 3
All good.
In [22]: sum_series(6) Out[22]: 1 # WTF???
The issue is that:
Default Arguments get evaluated when the function is defined. So every time the function is called, it will use the same list! Each time adding more and more to the list.
Let’s explore that some more, and some solutions….
Recursion in an interactive loop¶
not a great idea!
you can do something like:
def mainloop():
while True:
ans = input("A question > ")
....
if ans == "again"
mainloop()
Let’s look at this:
examples/session04/recursive_mainloop.py
(do a git pull upstream master if you don’t see it.)