Learn 90% of Python in 90 Minutes

90% of Python in 90 Minutes
@__mharrison__
Copyright 2013

About Me
●

●

●

12+ years Python
Worked in Data Analysis, HA, Search,
Open Source, BI, and Storage
Author of multiple Python Books

Goal
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Read Python

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Write Python

Book
Treading on Python Volume 1 covers this
talk in much more detail.

Disclaimer
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Assume some programming experience

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Not covering all api's, just syntax

Warning
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Starting from ground zero

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Hands-on at end

Three Python'isms to Remember
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dir

●

help

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colon/indent shuffle

Why Python?
Python is a powerful, multi-paradigm,
interpreted language popular with
start-ups and large Co’s

Python 2 or 3?
For beginners there is no real difference
between Python 2 & 3. The basics are the
same (except for print)

hello world
print "hello world"

from interpreter
$ python
>>> print "hello world"
hello world

REPL
$ python
>>> 2 + 2
4
>>>

#
#
#

read, eval
print
repeat (loop)

REPL (2)
Many developers keep a REPL handy
during programming

From script
Make file hello.py with
print "hello world"
Run with:
python hello.py

(unix) script
Make file hello with
#!/usr/bin/env python
print "hello world"
Run with:
chmod +x hello
./hello

Python 3 hello world
print is no longer a statement, but a
function
print("hello world")

Objects
Everything in Python is an object that has:
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an identity (id)

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a value (mutable or immutable)

id
>>> a = 4
>>> id(a)
6406896

Value
●

●

Mutable:When you alter the item, the
id is still the same. Dictionary, List
Immutable:String, Integer, Tuple

Mutable
>>> b = []
>>> id(b)
140675605442000
>>> b.append(3)
>>> b
[3]
>>> id(b)
140675605442000

# SAME!

Immutable
>>> a = 4
>>> id(a)
6406896
>>> a = a + 1
>>> id(a)
6406872
# DIFFERENT!

Variables
a
b
c
a

=
=
=
=

4
# Integer
5.6
# Float
"hello" # String
"4"
# rebound to String

Naming
●

lowercase

●

underscore_between_words

●

don't start with numbers

See PEP 8

PEP
Python Enhancement Proposal (similar to
JSR in Java)

Math
+, -, *, /, ** (power), % (modulo)

Careful with integer division
>>> 3/4
0
>>> 3/4.
0.75
(In Python 3 // is integer division
operator)

What happens when you
raise 10 to the 100th?

Long
>>> 10**100
10000000000000000000000000000000
00000000000000000000000000000000
00000000000000000000000000000000
00000L

Long (2)
>>> import sys
>>> sys.maxint
9223372036854775807
>>> sys.maxint + 1
9223372036854775808L

Strings
name = 'matt'
with_quote = "I ain't gonna"
longer = """This string has
multiple lines
in it"""

How do I print?
He said, “I’m sorry”

String escaping
Escape with
>>> print 'He said, "I'm sorry"'
He said, "I'm sorry"
>>> print '''He said, "I'm sorry"'''
>>> print """He said, "I'm sorry""""

String escaping (2)
Escape Sequence

Output

Backslash

'

Single quote

"

Double quote

b

ASCII Backspace

n

Newline

t

Tab

u12af

Unicode 16 bit

U12af89bc

Unicode 32 bit

o84

Octal character

xFF

Hex character

String formatting
c-like
>>> "%s %s" %('hello', 'world')
'hello world'

PEP 3101 style
>>> "{0} {1}".format('hello', 'world')
'hello world'

dir
Lists attributes and methods:
>>> dir("a string")
['__add__', '__class__', ... 'startswith',
'swapcase', 'title', 'translate', 'upper',

'strip',
'zfill']

Whats with all the
'__blah__'?

dunder methods
dunder (double under) or "special/magic"
methods determine what will happen
when + (__add__) or / (__div__) is
called.

help
>>> help("a string".startswith)
Help on built-in function startswith:
startswith(...)
S.startswith(prefix[, start[, end]]) -> bool
Return True if S starts with the specified prefix, False
otherwise.
With optional start, test S beginning at that position.
With optional end, stop comparing S at that position.
prefix can also be a tuple of strings to try.

String methods
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s.endswith(sub)
Returns True if endswith sub

●

s.find(sub)
Returns index of sub or -1

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s.format(*args)
Places args in string

String methods (2)
●

s.index(sub)
Returns index of sub or exception

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s.join(list)
Returns list items separated by string

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s.strip()
Removes whitespace from start/end

comments
No multi-line comments

None
Pythonic way of saying NULL. Evaluates
to False.
c = None

sequences
●

lists

●

tuples

●

sets

lists
Hold sequences.
How would we find out the attributes &
methods of a list?

lists
>>> dir([])
['__add__', '__class__', '__contains__',...
'__iter__',... '__len__',... , 'append', 'count',
'extend', 'index', 'insert', 'pop', 'remove',
'reverse', 'sort']

lists
>>>
>>>
>>>
>>>
>>>
>>>
[1,

a = []
a.append(4)
a.append('hello')
a.append(1)
a.sort() # in place
print a
4, 'hello']

lists
How would we find out documentation
for a method?

lists
help function:
>>> help([].append)
Help on built-in function append:
append(...)
L.append(object) -- append object to end

List methods
●

l.append(x)
Insert x at end of list

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l.extend(l2)
Add l2 items to list

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l.sort()
In place sort

List methods (2)
●

l.reverse()
Reverse list in place

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l.remove(item)
Remove first item found

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l.pop()
Remove/return item at end of list

dictionaries
Also called hashmap or associative array
elsewhere
>>>
>>>
>>>
>>>
>>>
10

age = {}
age['george'] = 10
age['fred'] = 12
age['henry'] = 10
print age['george']

dictionaries (2)
Find out if 'matt' in age
>>> 'matt' in age
False

in statement
Uses __contains__ dunder method to
determine membership. (Or __iter__ as
fallback)

.get
>>> print age['charles']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'charles'
>>> print age.get('charles', 'Not found')
Not found

deleting keys
Removing 'charles' from age
>>> del age['charles']

deleting keys
del not in dir. .pop is an alternative

functions
def add_2(num):
""" return 2
more than num
"""
return num + 2
five = add_2(3)

functions (2)
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def

●

function name

●

(parameters)

●

: + indent

●

optional documentation

●

body

●

return

whitespace
Instead of { use a : and indent
consistently (4 spaces)

whitespace (2)
invoke python -tt to error out during
inconsistent tab/space usage in a file

default (named) parameters
def add_n(num, n=3):
"""default to
adding 3"""
return num + n
five = add_n(2)
ten = add_n(15, -5)

__doc__
Functions have docstrings. Accessible
via .__doc__ or help

__doc__
>>> def echo(txt):
...
"echo back txt"
...
return txt
>>> help(echo)
Help on function echo in module __main__:
<BLANKLINE>
echo(txt)
echo back txt
<BLANKLINE>

naming
●

lowercase

●

underscore_between_words

●


●

verb

See PEP 8

conditionals
if grade > 90:
print "A"
elif grade > 80:
print "B"
elif grade > 70:
print "C"
else:
print "D"

Remember the
colon/whitespace!

Comparison Operators
Supports (>, >=, <, <=, ==, !=)
>>> 5 > 9
False
>>> 'matt' != 'fred'
True
>>> isinstance('matt',
basestring)
True

Boolean Operators
and, or, not (for logical), &, |, and ^ (for
bitwise)
>>> x = 5
>>> x < -4 or x > 4
True

Boolean note
Parens are only required for precedence
if (x > 10):
print "Big"
same as
if x > 10:
print "Big"

Chained comparisons
if 3 < x < 5:
print "Four!"
Same as
if x > 3 and x < 5:
print "Four!"

iteration
for number in [1,2,3,4,5,6]:
print number
for number in range(1, 7):
print number

range note
Python tends to follow half-open interval
([start,end)) with range and slices.
●

end - start = length

●

easy to concat ranges w/o overlap

iteration (2)
Java/C-esque style of object in array
access (BAD):
animals = ["cat", "dog", "bird"]
for index in range(len(animals)):
print index, animals[index]

iteration (3)
If you need indices, use enumerate
animals = ["cat", "dog", "bird"]
for index, value in enumerate(animals):
print index, value

iteration (4)
Can break out of nearest loop
for item in sequence:
# process until first negative
if item < 0:
break
# process item

iteration (5)
Can continue to skip over items
for item in sequence:
if item < 0:
continue
# process all positive items

iteration (6)
Can loop over lists, strings, iterators,
dictionaries... sequence like things:
my_dict = { "name": "matt", "cash": 5.45}
for key in my_dict.keys():
# process key
for value in my_dict.values():
# process value
for key, value in my_dict.items():
# process items

pass
pass is a null operation
for i in
# do
pass

range(10):
nothing 10 times

Hint
Don't modify list or dictionary contents
while looping over them

Slicing
Sequences (lists, tuples, strings, etc) can
be sliced to pull out a single item
my_pets = ["dog", "cat", "bird"]
favorite = my_pets[0]
bird = my_pets[-1]

Negative Indexing
Proper way to think of [negative
indexing] is to reinterpret a[-X] as
a[len(a)-X]
@gvanrossum

Slicing (2)
Slices can take an end index, to pull out a
list of items
# a list
cat_and_dog = my_pets[0:2]
cat_and_dog2 = my_pets[:2]
cat_and_bird = my_pets[1:3]
cat_and_bird2 = my_pets[1:]

Slicing (3)
Slices can take a stride
# a list
dog_and_bird = [0:3:2]
zero_three_etc = range(0,10)
[::3]

Slicing (4)
Just to beat it in
veg = "tomatoe"
correct = veg[:-1]
tmte = veg[::2]
eotamot = veg[::-1]

File Input
Open a file to read from it (old style):
fin = open("foo.txt")
for line in fin:
# manipulate line
fin.close()

File Output
Open a file using 'w' to write to a file:
fout = open("bar.txt", "w")
fout.write("hello world")
fout.close()

Always remember to
close your files!

closing with with
implicit close (new 2.5+ style)
with open('bar.txt') as fin:
for line in fin:
# process line

Classes
class Animal(object):
def __init__(self, name):
self.name = name
def talk(self):
print "Generic Animal Sound"
animal = Animal("thing")
animal.talk()

Classes (2)
notes:
●

object (base class) (fixed in 3.X)

●

dunder init (constructor)

●

all methods take self as first parameter

Classes(2)
Subclassing
class Cat(Animal):
def talk(self):
print '%s says, "Meow!"' % (self.name)
cat = Cat("Groucho")
cat.talk() # invoke method

Classes(3)
class Cheetah(Cat):
"""classes can have
docstrings"""
def talk(self):
print "Growl"

naming
●

CamelCase

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●

Nouns

Poor mans
print works a lot of the time

Remember
Clean up print statements. If you really
need them, use logging or write to
sys.stdout

pdb
import pdb; pdb.set_trace()

pdb commands
●

h - help

●

s - step into

●

n - next

●

c - continue

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w - where am I (in stack)?

●

l - list code around me

That's all
Questions? Tweet me
For more details see
Treading on Python Volume 1
@__mharrison__
http://hairysun.com

Learn 90% of Python in 90 Minutes

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