Notes for Session 04¶
April 30, 2019¶
NOTE: These notes are “live” – they will change up until the class starts..
A collection of notes to go over in class, to keep things organized.
Issues that came up during the week.¶
Indexing vs Slicing¶
A few of you ran into this issue in the slicing lab:
If you have a sequence, say a tuple, like this:
(5, 3, 8, 12, 45, 13, 5)
What does an index return?
This comes up in the slicing lab: exchange the first and last items of the sequence:
Let’s check it out. In the class repo:
SP_2019_210A_classroom/examples/Session04/index_vs_slice.py
(git pull upstream master)
Built in names¶
Python has a number of “key words” that are reserved. If you try to use them as a variable name, you will get an error:
In [1]: for = 5
File "<ipython-input-1-36df406aa65d>", line 1
for = 5
^
SyntaxError: invalid syntax
Sometimes a bit confusing, as it’s not REALLY a syntax error, but rather using a keyword as a variable….
But there are also a LOT of “built in” names. Try:
dir(__builtins__)
You can use these names for variable, but when you do, it willwrite over the built-in one, which means that you then can’t use it in the udual way. For instance, a list is the list type, you can make a list out of any sequence with it:
In [6]: list("this")
Out[6]: ['t', 'h', 'i', 's']
But if you use it as a name, it then won’t work in the usual way:
In [7]: list = [1,2,3,4,5]
In [8]: list
Out[8]: [1, 2, 3, 4, 5]
In [9]: list("this")
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-9-c89f7438771a> in <module>()
----> 1 list("this")
TypeError: 'list' object is not callable
Moral of the story:
Don’t “shadow” built in names
Use things like:
my_list
infile
or even misspellings or adding an underscore:
klass or cls or _class instead of class
Hopefully your editor will warn you – otherwise, think about it when you get a strange error like:
In [14]: input = "this was some input"
In [15]: input("=>")
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-15-56a225daeb09> in <module>()
----> 1 input("=>")
TypeError: 'str' object is not callable
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. Someone 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?
(examples\session04\series_with_mutable.py)
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.)
String formatting¶
Here’s an example:
tup3 = tuple(range(10))
print("{:5} {:5} {:5} {:5} {:5} {:5} {:5} {:5} {:5} {:5}".format(*tup3))
Seems like a lot of typing to me :-)
Slicing and List labs¶
Any questions?
Altering a list while looping through it¶
what could go wrong with this code?
for i in a_list:
if some_condition:
a_list.remove(i)
Let’s try it out …
examples/session04/deleting_in_loop.py
My solutions¶
Let’s look at my solutions quickly.
Lightning Talks:¶
Let’s take a break and do them…