11/02/2020: A actual Program!¶
A collection of notes to go over in class, to keep things organized.
Lightning Talks¶
As usual, these notes are published here:
https://uwpce-pythoncert-classrepos.github.io/Python310-Fall-2020/
And the Solutions are in teh gitHub project:
https://github.com/UWPCE-PythonCert-ClassRepos/Python310-Fall-2020
Quick Poll: how many of you have a clone of that repo on your machine?
Issues that came up during the week.¶
Style¶
Use PEP8 style – really!
https://www.python.org/dev/peps/pep-0008/
The ONLY exception is if you work in an organization that has a different style guide. It can make sense for your python code to match other code in an organization. But otherwise, use a style consistent with the rest of the Python world.
The best way to do this is with a linter in your editor – like the Anaconda package in Sublime. If you are getting annoyed by all the “noise” that the linter creates – keep your code in PEP8 style, it won’t be there!
And don’t use “Hungarian Notation” – it is really non-pythonic, and sometimes actually wrong – and a string called intSomething just adds confusion!
Also: use meaningful names, not “item” or “tup” or ….
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 a variable, but when you do, it will write over the built-in one, which means that you then can’t use it in the usual way. For instance, 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 …
But if you are confused ….
This is a real “gotcha” in Python. Someone (in a previous class) 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\lesson04\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/lesson04/recursive_mainloop.py
(do a git pull if you have a clone of the class repo)
Lightning Talks¶
let’s take a break, and then:
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/lesson04/deleting_in_loop.py
String Formatting¶
Are you starting to get it? There is a LOT there. But it lets you do some fancy stuff with not much code.
Lightning Talks¶
New Material¶
Any questions on
Unit Testing or Exceptions?
(we’ll start some in class)
Unit Testing and TDD¶
Compared to most Python courses, we are introducing unit testing really early. In fact, earlier that we used to in this class, which was already early.
But it is a REALLY good habit to get into, and, in fact, can help beginners just as much (maybe more) than more experienced developers.
Exceptions¶
Let’s do the Exceptions Lab in class:
https://uwpce-pythoncert.github.io/ProgrammingInPython/exercises/exceptions/exceptions_lab.html
Testing Mailroom¶
Key point here:
You will need to refactor your code in order to make it testable.
That is a “Good Thing”
Well structured code is testable : testable code is (Probably) well structured.
This is one reason we’re introducing unit tests right now.
Shall we get a start on that together?