Appendix: Python Language Essentials¶

from __future__ import division
from numpy.random import randn
import numpy as np
import os
import matplotlib.pyplot as plt
np.random.seed(12345)
plt.rc('figure', figsize=(10, 6))
from pandas import *
import pandas
np.set_printoptions(precision=4)

%%writefile hello_world.py
print 'Hello world'

results = []
for line in file_handle:
    # keep the empty lines for now
    # if len(line) == 0:
    #   continue
    results.append(line.replace('foo', 'bar'))

result = f(x, y, z)
g()

obj.some_method(x, y, z)

result = f(a, b, c, d=5, e='foo')

a = [1, 2, 3]

b = a

a.append(4)
b

def append_element(some_list, element):
    some_list.append(element)

data = [1, 2, 3]

append_element(data, 4)

In [4]: data
Out[4]: [1, 2, 3, 4]

a = 5
type(a)
a = 'foo'
type(a)

'5' + 5

a = 4.5
b = 2
# String formatting, to be visited later
print 'a is %s, b is %s' % (type(a), type(b))
a / b

a = 5
isinstance(a, int)

a = 5; b = 4.5
isinstance(a, (int, float))
isinstance(b, (int, float))

def isiterable(obj):
    try:
        iter(obj)
        return True
    except TypeError: # not iterable
        return False

isiterable('a string')
isiterable([1, 2, 3])
isiterable(5)

# some_module.py
PI = 3.14159

def f(x):
    return x + 2

def g(a, b):
    return a + b

import some_module
result = some_module.f(5)
pi = some_module.PI

from some_module import f, g, PI
result = g(5, PI)

import some_module as sm
from some_module import PI as pi, g as gf

r1 = sm.f(pi)
r2 = gf(6, pi)

5 - 7
12 + 21.5
5 <= 2

a = [1, 2, 3]
b = a
# Note, the list function always creates a new list
c = list(a)
a is b
a is not c

a == c

a = None
a is None

a = b = c = 5
d = a + b * c

a_list = ['foo', 2, [4, 5]]
a_list[2] = (3, 4)
a_list

a_tuple = (3, 5, (4, 5))
a_tuple[1] = 'four'

ival = 17239871
ival ** 6

fval = 7.243
fval2 = 6.78e-5

3 / 2

from __future__ import division

3 / float(2)

3 // 2

cval = 1 + 2j
cval * (1 - 2j)

a = 'one way of writing a string'
b = "another way"

c = """
This is a longer string that
spans multiple lines
"""

a = 'this is a string'
a[10] = 'f'
b = a.replace('string', 'longer string')
b

a = 5.6
s = str(a)
s

s = 'python'
list(s)
s[:3]

s = '12\\34'
print s

s = r'this\has\no\special\characters'
s

a = 'this is the first half '
b = 'and this is the second half'
a + b

template = '%.2f %s are worth $%d'

template % (4.5560, 'Argentine Pesos', 1)

True and True
False or True

a = [1, 2, 3]
if a:
    print 'I found something!'

b = []
if not b:
    print 'Empty!'

bool([]), bool([1, 2, 3])
bool('Hello world!'), bool('')
bool(0), bool(1)

s = '3.14159'
fval = float(s)
type(fval)
int(fval)
bool(fval)
bool(0)

a = None
a is None
b = 5
b is not None

def add_and_maybe_multiply(a, b, c=None):
    result = a + b

    if c is not None:
        result = result * c

    return result

from datetime import datetime, date, time
dt = datetime(2011, 10, 29, 20, 30, 21)
dt.day
dt.minute

dt.date()
dt.time()

dt.strftime('%m/%d/%Y %H:%M')

datetime.strptime('20091031', '%Y%m%d')

dt.replace(minute=0, second=0)

dt2 = datetime(2011, 11, 15, 22, 30)
delta = dt2 - dt
delta
type(delta)

dt
dt + delta

if x < 0:
    print 'It's negative'

if x < 0:
    print 'It's negative'
elif x == 0:
    print 'Equal to zero'
elif 0 < x < 5:
    print 'Positive but smaller than 5'
else:
    print 'Positive and larger than 5'

a = 5; b = 7
c = 8; d = 4
if a < b or c > d:
    print 'Made it'

for value in collection:
    # do something with value

sequence = [1, 2, None, 4, None, 5]
total = 0
for value in sequence:
    if value is None:
        continue
    total += value

sequence = [1, 2, 0, 4, 6, 5, 2, 1]
total_until_5 = 0
for value in sequence:
    if value == 5:
        break
    total_until_5 += value

for a, b, c in iterator:
    # do something

x = 256
total = 0
while x > 0:
    if total > 500:
        break
    total += x
    x = x // 2

if x < 0:
    print 'negative!'
elif x == 0:
    # TODO: put something smart here
    pass
else:
    print 'positive!'

def f(x, y, z):
    # TODO: implement this function!
    pass

float('1.2345')
float('something')

def attempt_float(x):
    try:
        return float(x)
    except:
        return x

attempt_float('1.2345')
attempt_float('something')

float((1, 2))

def attempt_float(x):
    try:
        return float(x)
    except ValueError:
        return x

attempt_float((1, 2))

def attempt_float(x):
    try:
        return float(x)
    except (TypeError, ValueError):
        return x

f = open(path, 'w')

try:
    write_to_file(f)
finally:
    f.close()

f = open(path, 'w')

try:
    write_to_file(f)
except:
    print 'Failed'
else:
    print 'Succeeded'
finally:
    f.close()

range(10)

range(0, 20, 2)

seq = [1, 2, 3, 4]
for i in range(len(seq)):
    val = seq[i]

sum = 0
for i in xrange(10000):
    # % is the modulo operator
    if i % 3 == 0 or i % 5 == 0:
        sum += i

x = 5
value = 'Non-negative' if x >= 0 else 'Negative'

tup = 4, 5, 6
tup

nested_tup = (4, 5, 6), (7, 8)
nested_tup

tuple([4, 0, 2])
tup = tuple('string')
tup

tup[0]

tup = tuple(['foo', [1, 2], True])
tup[2] = False

# however
tup[1].append(3)
tup

(4, None, 'foo') + (6, 0) + ('bar',)

('foo', 'bar') * 4

tup = (4, 5, 6)
a, b, c = tup
b

tup = 4, 5, (6, 7)
a, b, (c, d) = tup
d

a = (1, 2, 2, 2, 3, 4, 2)
a.count(2)

a_list = [2, 3, 7, None]

tup = ('foo', 'bar', 'baz')
b_list = list(tup)
b_list
b_list[1] = 'peekaboo'
b_list

b_list.append('dwarf')
b_list

b_list.insert(1, 'red')
b_list

b_list.pop(2)
b_list

b_list.append('foo')
b_list.remove('foo')
b_list

'dwarf' in b_list

[4, None, 'foo'] + [7, 8, (2, 3)]

x = [4, None, 'foo']
x.extend([7, 8, (2, 3)])
x

everything = []
for chunk in list_of_lists:
    everything.extend(chunk)

everything = []
for chunk in list_of_lists:
    everything = everything + chunk

a = [7, 2, 5, 1, 3]
a.sort()
a

b = ['saw', 'small', 'He', 'foxes', 'six']
b.sort(key=len)
b

import bisect
c = [1, 2, 2, 2, 3, 4, 7]
bisect.bisect(c, 2)
bisect.bisect(c, 5)
bisect.insort(c, 6)
c

seq = [7, 2, 3, 7, 5, 6, 0, 1]
seq[1:5]

seq[3:4] = [6, 3]
seq

seq[:5]
seq[3:]

seq[-4:]
seq[-6:-2]

seq[::2]

seq[::-1]

i = 0
for value in collection:
   # do something with value
   i += 1

for i, value in enumerate(collection):
   # do something with value

some_list = ['foo', 'bar', 'baz']
mapping = dict((v, i) for i, v in enumerate(some_list))
mapping

sorted([7, 1, 2, 6, 0, 3, 2])
sorted('horse race')

sorted(set('this is just some string'))

seq1 = ['foo', 'bar', 'baz']
seq2 = ['one', 'two', 'three']
zip(seq1, seq2)

seq3 = [False, True]
zip(seq1, seq2, seq3)

for i, (a, b) in enumerate(zip(seq1, seq2)):
    print('%d: %s, %s' % (i, a, b))

pitchers = [('Nolan', 'Ryan'), ('Roger', 'Clemens'),
            ('Schilling', 'Curt')]
first_names, last_names = zip(*pitchers)
first_names
last_names

zip(seq[0], seq[1], ..., seq[len(seq) - 1])

list(reversed(range(10)))

empty_dict = {}
d1 = {'a' : 'some value', 'b' : [1, 2, 3, 4]}
d1

d1[7] = 'an integer'
d1
d1['b']

'b' in d1

d1[5] = 'some value'
d1['dummy'] = 'another value'
del d1[5]
ret = d1.pop('dummy')
ret

d1.keys()
d1.values()

d1.update({'b' : 'foo', 'c' : 12})
d1

mapping = {}
for key, value in zip(key_list, value_list):
    mapping[key] = value

mapping = dict(zip(range(5), reversed(range(5))))
mapping

if key in some_dict:
    value = some_dict[key]
else:
    value = default_value

value = some_dict.get(key, default_value)

words = ['apple', 'bat', 'bar', 'atom', 'book']
by_letter = {}

for word in words:
    letter = word[0]
    if letter not in by_letter:
        by_letter[letter] = [word]
    else:
        by_letter[letter].append(word)

by_letter

from collections import defaultdict
by_letter = defaultdict(list)
for word in words:
    by_letter[word[0]].append(word)

counts = defaultdict(lambda: 4)

hash('string')
hash((1, 2, (2, 3)))
hash((1, 2, [2, 3])) # fails because lists are mutable

d = {}
d[tuple([1, 2, 3])] = 5
d

set([2, 2, 2, 1, 3, 3])
{2, 2, 2, 1, 3, 3}

a = {1, 2, 3, 4, 5}
b = {3, 4, 5, 6, 7, 8}
a | b  # union (or)
a & b  # intersection (and)
a - b  # difference
a ^ b  # symmetric difference (xor)

a_set = {1, 2, 3, 4, 5}
{1, 2, 3}.issubset(a_set)
a_set.issuperset({1, 2, 3})

{1, 2, 3} == {3, 2, 1}

strings = ['a', 'as', 'bat', 'car', 'dove', 'python']
[x.upper() for x in strings if len(x) > 2]

unique_lengths = {len(x) for x in strings}
unique_lengths

loc_mapping = {val : index for index, val in enumerate(strings)}
loc_mapping

all_data = [['Tom', 'Billy', 'Jefferson', 'Andrew', 'Wesley', 'Steven', 'Joe'],
            ['Susie', 'Casey', 'Jill', 'Ana', 'Eva', 'Jennifer', 'Stephanie']]

names_of_interest = []
for names in all_data:
    enough_es = [name for name in names if name.count('e') > 2]
    names_of_interest.extend(enough_es)

result = [name for names in all_data for name in names
          if name.count('e') >= 2]
result

some_tuples = [(1, 2, 3), (4, 5, 6), (7, 8, 9)]
flattened = [x for tup in some_tuples for x in tup]
flattened

flattened = []

for tup in some_tuples:
    for x in tup:
        flattened.append(x)

In [229]: [[x for x in tup] for tup in some_tuples]

def my_function(x, y, z=1.5):
    if z > 1:
        return z * (x + y)
    else:
        return z / (x + y)

my_function(5, 6, z=0.7)
my_function(3.14, 7, 3.5)

def func():
    a = []
    for i in range(5):
        a.append(i)

a = []
def func():
    for i in range(5):
        a.append(i)

a = None
def bind_a_variable():
    global a
    a = []
bind_a_variable()
print a

def outer_function(x, y, z):
    def inner_function(a, b, c):
        pass
    pass

def f():
    a = 5
    b = 6
    c = 7
    return a, b, c

a, b, c = f()

return_value = f()

def f():
    a = 5
    b = 6
    c = 7
    return {'a' : a, 'b' : b, 'c' : c}

states = ['   Alabama ', 'Georgia!', 'Georgia', 'georgia', 'FlOrIda',
          'south   carolina##', 'West virginia?']

import re  # Regular expression module

def clean_strings(strings):
    result = []
    for value in strings:
        value = value.strip()
        value = re.sub('[!#?]', '', value) # remove punctuation
        value = value.title()
        result.append(value)
    return result

In [15]: clean_strings(states)
Out[15]:
['Alabama',
 'Georgia',
 'Georgia',
 'Georgia',
 'Florida',
 'South Carolina',
 'West Virginia']

def remove_punctuation(value):
    return re.sub('[!#?]', '', value)

clean_ops = [str.strip, remove_punctuation, str.title]

def clean_strings(strings, ops):
    result = []
    for value in strings:
        for function in ops:
            value = function(value)
        result.append(value)
    return result

In [22]: clean_strings(states, clean_ops)
Out[22]:
['Alabama',
 'Georgia',
 'Georgia',
 'Georgia',
 'Florida',
 'South Carolina',
 'West Virginia']

In [23]: map(remove_punctuation, states)
Out[23]:
['   Alabama ',
 'Georgia',
 'Georgia',
 'georgia',
 'FlOrIda',
 'south   carolina',
 'West virginia']

def short_function(x):
    return x * 2

equiv_anon = lambda x: x * 2

def apply_to_list(some_list, f):
    return [f(x) for x in some_list]

ints = [4, 0, 1, 5, 6]
apply_to_list(ints, lambda x: x * 2)

strings = ['foo', 'card', 'bar', 'aaaa', 'abab']

strings.sort(key=lambda x: len(set(list(x))))
strings

def make_closure(a):
    def closure():
        print('I know the secret: %d' % a)
    return closure

closure = make_closure(5)

def make_watcher():
    have_seen = {}

    def has_been_seen(x):
        if x in have_seen:
            return True
        else:
            have_seen[x] = True
            return False

    return has_been_seen

watcher = make_watcher()
vals = [5, 6, 1, 5, 1, 6, 3, 5]
[watcher(x) for x in vals]

def format_and_pad(template, space):
    def formatter(x):
        return (template % x).rjust(space)

    return formatter

fmt = format_and_pad('%.4f', 15)
fmt(1.756)

a, b, c = args
d = kwargs.get('d', d_default_value)
e = kwargs.get('e', e_default_value)

def say_hello_then_call_f(f, *args, **kwargs):
    print 'args is', args
    print 'kwargs is', kwargs
    print("Hello! Now I'm going to call %s" % f)
    return f(*args, **kwargs)

def g(x, y, z=1):
    return (x + y) / z

In [8]:  say_hello_then_call_f(g, 1, 2, z=5.)
args is (1, 2)
kwargs is {'z': 5.0}
Hello! Now I'm going to call <function g at 0x2dd5cf8>
Out[8]: 0.6

def add_numbers(x, y):
    return x + y

add_five = lambda y: add_numbers(5, y)

from functools import partial
add_five = partial(add_numbers, 5)

# compute 60-day moving average of time series x
ma60 = lambda x: pandas.rolling_mean(x, 60)

# Take the 60-day moving average of of all time series in data
data.apply(ma60)

some_dict = {'a': 1, 'b': 2, 'c': 3}
for key in some_dict:
    print key,

dict_iterator = iter(some_dict)
dict_iterator

list(dict_iterator)

def squares(n=10):
    for i in xrange(1, n + 1):
        print 'Generating squares from 1 to %d' % (n ** 2)
        yield i ** 2

In [2]: gen = squares()

In [3]: gen
Out[3]: <generator object squares at 0x34c8280>

In [4]: for x in gen:
   ...:     print x,
   ...:
Generating squares from 0 to 100
1 4 9 16 25 36 49 64 81 100

def make_change(amount, coins=[1, 5, 10, 25], hand=None):
    hand = [] if hand is None else hand
    if amount == 0:
        yield hand
    for coin in coins:
        # ensures we don't give too much change, and combinations are unique
        if coin > amount or (len(hand) > 0 and hand[-1] < coin):
            continue

        for result in make_change(amount - coin, coins=coins,
                                  hand=hand + [coin]):
            yield result

for way in make_change(100, coins=[10, 25, 50]):
    print way
len(list(make_change(100)))

gen = (x ** 2 for x in xrange(100))
gen

def _make_gen():
    for x in xrange(100):
        yield x ** 2
gen = _make_gen()

sum(x ** 2 for x in xrange(100))
dict((i, i **2) for i in xrange(5))

import itertools
first_letter = lambda x: x[0]

names = ['Alan', 'Adam', 'Wes', 'Will', 'Albert', 'Steven']

for letter, names in itertools.groupby(names, first_letter):
    print letter, list(names) # names is a generator

path = 'ch13/segismundo.txt'
f = open(path)

for line in f:
    pass

lines = [x.rstrip() for x in open(path)]
lines

with open('tmp.txt', 'w') as handle:
    handle.writelines(x for x in open(path) if len(x) > 1)

open('tmp.txt').readlines()

os.remove('tmp.txt')