Notes for Session 05¶
May 7, 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.¶
git and generated files¶
In general, you don’t want to put generated files in git.
In this case, the letters your mailroom program created.
Caution: be very careful with git add . or git add * – generally better to specifically add the files you now you need.
‘.gitignore’ can help here.
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.
And don’t use “Hungarian Notation” – it is really non-pythonic, and sometimes actually wrong – and a string called intSomething just adds confusion!
The best way to do this is with a linter in your editor – like the Anaconda package in Sublime. If there is no way to get a Python linter in your editor/IDE – find another one :-)
Minor Issues¶
Looping through dicts¶
dicts are a bit odd, in that they are kind of two sequences – they keys and the values. Sometimes you want one, sometimes the other, sometimes both.
The keys are kind of the “first class” things when used with the regular sequence protocol.
So:
something in a_dict checks if this is a key.
Similarly:
for k in dict:
Loops through the keys. So no need for:
for k in dict.keys():
But for the values:
v in a_dict.values()
and
for v in a_dict.values()
For both:
for a_key, value in a_dict.items()
Getting an arbitrary key from a dict¶
See arbitrary_key.py in examples/session05
Nifty formatting¶
what the heck is this?
def data_print(info, widths):
"""
takes in donor information and widths and returns a string formatted for
printing for a donor report.
"""
output_string = ""
output_string += '{:<{width0}} ${:>{width1}.2f} {:^{width2}} ${:>{width3}}'.format(info[0], info[1], info[2], info[3], width0=widths[0], width1=widths[1]-1, width2=widths[2], width3=widths[3]-1)
cleaned up a bit:
'{:<{widths[0]}} ${:>{widths[1]}.2f}'.format( "fred", 100, widths=widths)
islice¶
This construct is pretty cool for trigrams:
for w1, w2, w3 in zip(word_list[:-2], word_list[1:-1], word_list[2:]):
But remember that slicing makes a copy – so this is making three copies of the full word list. Computers have a LOT of memory these days, but it’s still better to not waste it.
Turns out there is a alternative:
https://docs.python.org/3.7/library/itertools.html#itertools.islice
How would we write that with islice?
Shall we do it?
dict.setdefault()¶
I don’t think anyone discovered this, but this construct:
if a_key in a_dict:
do_something_with(a_dict[a_key])
else:
a_dict[a_key] = something
do_something_with(something)
seemed like a common thing to want to do – so I thought for years that there should be a better way – turn out there is!
do_somethingwith(a_dict.setdefault(a_key, something))
For a concrete example, let’s look at trigrams:
Coding Workflow¶
As you are developing your code, you really want to have an quick and efficient way to run you code and see if it’s working, how it’s changed, etc.
You may have noticed that for a program like Mailroom, you may have to do a few steps if user interaction to get to the part of the code you are working on. So how do you work on that efficiently?
The “right” way to do it is something called “Test Driven Development”, which we will get to soon. But in the meantime:
- You want to break your code down into small functions that each do one thing.
- You should be able to run each function by itself.
If you are doing that, then as you develop your code, you can write and run each function until it’s doing what it’s supposed to do, and THEN put it all together.
One way to run a function is to call it in the __name__ == "__main__" block. You can then comment and uncomment each call as you work on your code.
Also: you really, really need a way to run your code with a couple keystrokes!!
I’ll demonstrate this when we review code.
Review of last week’s assignments¶
Mailroom review¶
Anyone up for a review?
Trigrams review¶
Anyone want to look at theirs?
Lightning Talks¶
Let’s take a break and do them now.
New Assignments¶
Comprehensions¶
Let’s take a few minutes to go through it in class:
https://uwpce-pythoncert.github.io/PythonCertDevel/exercises/comprehensions_lab.html
Exceptions¶
Exceptions take a little while to “wrap your head around”.
Shall we do the Exercise together?
https://uwpce-pythoncert.github.io/PythonCertDevel/exercises/except_exercise.html