Notes for Session 06¶
May 14, 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.
So I don’t forget!!!¶
I (Chris) will be out of town this weekend, next week, and the following weekend: Bryan will be covering class next week, and hopefully office hours over the weekends. Look for info from him about that.
I’ll be back the following week, then off again, so we’re starting to transition to Bryan for the rest of the class.
mailroom 4!!!!¶
WTF?
What if I haven’t gotten version 2 done yet???
It all builds on itself – so no need to do each intermediate step if you haven’t already. If you submit a well tested mailroom with some use of dictionaries and exception handling, we’ll give you credit for all of them.
Issues that came up during the week.¶
dict as switch – how do you leave the loop?¶
Let’s look at my solution:
solutions/Lesson05/mailroom3.py
quit()¶
In my solution to mailroom, I created a function called quit to quit the program. That is not a great idea, as there is a built-in called quit. In my defense, the quit() built-in didn’t exist when I learned Python :-).
but you can simply put the built-in quit in your switch dict.
readlines() ?¶
quite a few of you have code like this:
with open(filename, "r") as f:
full_lines = f.readlines()
for line in full_lines:
...
Nothing wrong with that, but …
.readlines() reads the entire contents of the file into memory all at once. Memory is big and cheap these days, but what if it’s a REALLY big file?
If you are going to process the file line by line anyway, you might as well do:
with open(filename, "r") as f:
for line in f:
...
That will loop though the file line by line, but only store one line at a time in memory. The file system and disk should have a smart cache, so that it will be just as fast, but more memory efficient.
And one less line of code :-)
binary vs text files¶
open() uses text mode (utf-8) by default. It will try to decode the file into text. If you open a binary file that way it will likely barf.
try: open(the_filename, 'rb'
For more on what “decode” means:
multiple values from a comprehension?¶
A few folks solved one the comp excercises this way:
dict(zip([number for number in range(start, stop + 1)], [hex(number) for number in range(start, stop + 1)]))
But …. I see two range() calls in there, that seems wasteful.
Do you need that?
You DO want a list (iterable) of tuples with the keys and values of the dict.
What does zip() actually do?
Can you do that “on the fly” in the comprehension?
Hint: yes.
And do we need a list here at all? maybe a generator comprehension instead?
Exception Handling¶
Key point: Always catch your exceptions as close as you can to the source.
Example: Miguel’s code:
students/roviram/Lesson05/mailroom_part3.py
Any Questions about the homework – or anything?¶
Let’s take a look at a comprehensions FizzBuzz:
solutions/Lesson05/fizz_buzz_comprehension.py
review trigrams?
review mailroom?