#lp5e

[babydatajournalism]

Python statements and syntax

Chapters 10 and 11 in Learning Python, 5th edition are the first in section 3, "Statements and Syntax."

"[Y]ou can make a single statement span across multiple lines. To make this work, you simply have to enclose part of your statement in a bracketed pair—parentheses (()), square brackets ([]), or curly braces ({}). Any code enclosed in these constructs can cross multiple lines ... Parentheses are the catchall device ..." p. 32

"An older rule also allows for continuation lines when the prior line ends in a backslash ... This alternative technique is dated, though, and is frowned on today because it’s difficult to notice and maintain the backslashes." (my bold)

Introducing the try statement:

while True: reply = raw_input('Enter text: ') if reply == 'stop': break try: num = int(reply) except: print 'Bad! ' * 8 else: print num ** 2 print 'Bye'

Chapter 10 is quite short! Just if, while, and try statements.

Assignments, expressions, and print are covered in chapter 11.

"Tuple assignment leads to a common coding trick in Python":

>>> nudge = 'NUDGE' >>> wink = 'WINK' >>> nudge, wink = wink, nudge >>> nudge 'WINK' >>> wink 'NUDGE'

"[W]e must generally have the same number of items on the right as we have variables on the left" -- not surprising, but this is an example of how clearly this book explains most things.

This is funky -- I had to think it out (p. 344):

>>> L = [1, 2, 3, 4] >>> while L: ... front, L = L[0], L[1:] ... print(front, L) ... 1 [2, 3, 4] 2 [3, 4] 3 [4] 4 []

Reserved words: "because module names in import statements become variables in your scripts, variable name constraints extend to your module filenames too."

Note: Names that begin with a single underscore (_X) are NOT imported by a "from module import xyz" statement.

(Python filenames may begin with an underscore.)

Expression statements: Expressions are commonly used as statements in two situations:

calls to functions and methods For functions and methods that do not return a value.

printing values at the interactive prompt i.e. in Terminal, we don't need to write "print."

"[A]lthough expressions can appear as statements in Python, statements cannot be used as expressions."

The rest of chapter 11 is about the print statement, with thorough coverage for both Python 3.x and 2.x (yay!). This is a simple thing:

print x, y # adds space between print x + y # no space

"[W]hereas file write methods write strings to arbitrary files, print writes objects to the stdout stream by default, with some automatic formatting added." p. 358

[babydatajournalism]

Tuples, Files, and Everything Else - Python

Chapter 9 in Learning Python, 5th edition has this rather unintuitive title, but it's the end of the section about "Object Types and Operations," so it's a kind of catch-all. (Seems to me the tuples could have gone in the previous chapter with lists and dictionaries.)

Tuples: "They work exactly like lists, except that tuples can’t be changed in place (they’re immutable) and are usually written as a series of items in parentheses, not square brackets." (p. 276)

You CAN reassign the variable name of a tuple, however -- like with everything in Python. So you could have T = (1, 2, 3) and then later change it, with T = (4, 5, 6).

You can change a tuple to a list: L = list(T) 

Or even T = list(T)

"Tuples can also be used in places that lists cannot — for example, as dictionary keys ...", which seems useful.

Files: Zed gave me a lot of practice with files (writing, reading) in LPTHW. Not much new on that score.

"Python also includes advanced standard library tools for handling generic object storage (the pickle module), for dealing with packed binary data in files (the struct module), and for processing special types of content such as JSON, XML, and CSV text." (p. 284)

JSON - Python standard library module http://docs.python.org/2.7/library/json.html

Python XML and CSV modules: http://docs.python.org/2/library/xml.html http://docs.python.org/2/library/csv.html

Then there's a set of review questions (pp. 313-315). Good review. I keep forgetting how index works:

L.index(4) means "get the index number of the number 4 in the list L."

>>> L = ['apple', 'peach', 'pear', 'strawberry'] >>> L.index('pear') 2 >>> L[2] 'pear'

New files added to my python_notes repo:

string_searches.py

write_read_file.py

I didn't write a new file for demonstrating pickle, because I already did that in useful_modules.py (3 weeks ago).

[babydatajournalism]

Chapters 5, 6, 7: Learning Python, 5th edition

Some more skimming was possible, as chapter 5 covers numbers and chapter 7 covers strings, and I have been working with them for a while now. Of course, I also discovered new things in Lutz's very thorough chapters -- even though he admits he is not going over everything we can do with numbers or strings in Python.

>>> math.floor(-2.5) -3 >>> math.trunc(-2.5) -2

I also learned more about sets than I had ever known before. They can be useful for comparisons and for getting rid of duplicate items in a list.

Chapter 6 sings the praises of dynamic typing in Python, which gives us lots of flexibility -- the type of a variable is not fixed or set or even declared. The variable name points to an object, and the object has a type -- NOT the variable. Many Python texts refer to this property, and yet I have never before seen such a sensible and clear explanation of what it really is, how it really works.

You can't CHANGE a string (it is immutable). To achieve a similar effect, you create a new string — "by concatenating, slicing, running formatting expressions, or using a method call like replace — and then assigning the result back to the original variable name." (The difference is not really detectable in most cases!)

Immutables: numbers, strings, tuples, frozensets Mutables: lists, dictionaries, sets, bytearray

Lutz spends an amazingly LONG time talking about format expressions and format method calls and the difference between them. Thanks to Zed Shaw (LPTHW), I am very comfy with format strings (expressions). I was able to follow Lutz's explication, but it was LONG! (Did I say that already?)

If you are familiar with the format strings (%s, %d, %r, etc.), suffice to say there is a rather different way to do essentially all the same stuff, and it involves use of curly braces -- {} -- instead.

>>> s = "beam" >>> s.replace('ea', 'oo') 'boom' >>> s = s + " crash " >>> s 'beam crash ' >>> s.rstrip() 'beam crash' >>> s[5:] # slice 'crash ' >>> s.upper() 'BEAM CRASH ' >>> s.endswith('ash ') True >>> s = "0123456789" >>> s[::2] # two-step slice '02468' >>> s = "bumblebee" >>> s[::-1] # reverse order slice 'eebelbmub'

So ... these are not surprising things, but I do love Python!

Oh, and: object.method(arguments) ... s.find()

>>> breakfast = "We love to eat green eggs and ham." >>> result = breakfast.find('eggs') >>> result 21 # position of 'eggs' in breakfast

Love, love Python.

Book: Learning Python, 5th edition