Python Language Basics, IPython, and Jupyter Notebooks



In [ ]:

    
import numpy as np
np.random.seed(12345)
np.set_printoptions(precision=4, suppress=True)

The Python Interpreter

$ python
Python 3.6.0 | packaged by conda-forge | (default, Jan 13 2017, 23:17:12)
[GCC 4.8.2 20140120 (Red Hat 4.8.2-15)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 5
>>> print(a)
5

print('Hello world')

$ python hello_world.py
Hello world

$ ipython
Python 3.6.0 | packaged by conda-forge | (default, Jan 13 2017, 23:17:12)
Type "copyright", "credits" or "license" for more information.

IPython 5.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: %run hello_world.py
Hello world

In [2]:

IPython Basics

Running the IPython Shell



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import numpy as np
data = {i : np.random.randn() for i in range(7)}
data









    Out[1]:





{0: 0.163237556162501,
 1: -0.37040882274338893,
 2: -0.08088067273179514,
 3: -0.9098779171581127,
 4: 0.25760185356933984,
 5: 0.5749282673553658,
 6: -0.8597249109794647}

from numpy.random import randn data = {i : randn() for i in range(7)} print(data) {0: -1.5948255432744511, 1: 0.10569006472787983, 2: 1.972367135977295, 3: 0.15455217573074576, 4: -0.24058577449429575, 5: -1.2904897053651216, 6: 0.3308507317325902}

Running the Jupyter Notebook

$ jupyter notebook
[I 15:20:52.739 NotebookApp] Serving notebooks from local directory:
/home/wesm/code/pydata-book
[I 15:20:52.739 NotebookApp] 0 active kernels
[I 15:20:52.739 NotebookApp] The Jupyter Notebook is running at:
http://localhost:8888/
[I 15:20:52.740 NotebookApp] Use Control-C to stop this server and shut down
all kernels (twice to skip confirmation).
Created new window in existing browser session.

Tab Completion

In [1]: an_apple = 27

In [2]: an_example = 42

In [3]: an

In [3]: b = [1, 2, 3]

In [4]: b.

In [1]: import datetime

In [2]: datetime.

In [7]: datasets/movielens/

Introspection

In [8]: b = [1, 2, 3]

In [9]: b?
Type:       list
String Form:[1, 2, 3]
Length:     3
Docstring:
list() -> new empty list
list(iterable) -> new list initialized from iterable's items

In [10]: print?
Docstring:
print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

Prints the values to a stream, or to sys.stdout by default.
Optional keyword arguments:
file:  a file-like object (stream); defaults to the current sys.stdout.
sep:   string inserted between values, default a space.
end:   string appended after the last value, default a newline.
flush: whether to forcibly flush the stream.
Type:      builtin_function_or_method

def add_numbers(a, b):
    """
    Add two numbers together

    Returns
    -------
    the_sum : type of arguments
    """
    return a + b

In [11]: add_numbers?
Signature: add_numbers(a, b)
Docstring:
Add two numbers together

Returns
-------
the_sum : type of arguments
File:      <ipython-input-9-6a548a216e27>
Type:      function

In [12]: add_numbers??
Signature: add_numbers(a, b)
Source:
def add_numbers(a, b):
    """
    Add two numbers together

    Returns
    -------
    the_sum : type of arguments
    """
    return a + b
File:      <ipython-input-9-6a548a216e27>
Type:      function

In [13]: np.*load*?
np.__loader__
np.load
np.loads
np.loadtxt
np.pkgload

The %run Command

def f(x, y, z):
    return (x + y) / z

a = 5
b = 6
c = 7.5

result = f(a, b, c)

In [14]: %run ipython_script_test.py

In [15]: c
Out [15]: 7.5

In [16]: result
Out[16]: 1.4666666666666666

>>> %load ipython_script_test.py

    def f(x, y, z):
        return (x + y) / z

    a = 5
    b = 6
    c = 7.5

    result = f(a, b, c)

Interrupting running code

Executing Code from the Clipboard

x = 5
y = 7
if x > 5:
    x += 1

    y = 8

In [17]: %paste
x = 5
y = 7
if x > 5:
    x += 1

    y = 8
## -- End pasted text --

In [18]: %cpaste
Pasting code; enter '--' alone on the line to stop or use Ctrl-D.
:x = 5
:y = 7
:if x > 5:
:    x += 1
:
:    y = 8
:--

Terminal Keyboard Shortcuts

About Magic Commands

In [20]: a = np.random.randn(100, 100)

In [20]: %timeit np.dot(a, a)
10000 loops, best of 3: 20.9 µs per loop

In [21]: %debug?
Docstring:
::

  %debug [--breakpoint FILE:LINE] [statement [statement ...]]

Activate the interactive debugger.

This magic command support two ways of activating debugger.
One is to activate debugger before executing code.  This way, you
can set a break point, to step through the code from the point.
You can use this mode by giving statements to execute and optionally
a breakpoint.

The other one is to activate debugger in post-mortem mode.  You can
activate this mode simply running %debug without any argument.
If an exception has just occurred, this lets you inspect its stack
frames interactively.  Note that this will always work only on the last
traceback that occurred, so you must call this quickly after an
exception that you wish to inspect has fired, because if another one
occurs, it clobbers the previous one.

If you want IPython to automatically do this on every exception, see
the %pdb magic for more details.

positional arguments:
  statement             Code to run in debugger. You can omit this in cell
                        magic mode.

optional arguments:
  --breakpoint <FILE:LINE>, -b <FILE:LINE>
                        Set break point at LINE in FILE.

In [22]: %pwd
Out[22]: '/home/wesm/code/pydata-book

In [23]: foo = %pwd

In [24]: foo
Out[24]: '/home/wesm/code/pydata-book'

Matplotlib Integration

In [26]: %matplotlib
Using matplotlib backend: Qt4Agg

In [26]: %matplotlib inline

Python Language Basics

Language Semantics

Indentation, not braces

for x in array:
    if x < pivot:
        less.append(x)
    else:
        greater.append(x)

a = 5; b = 6; c = 7

Everything is an object

Comments

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

print("Reached this line")  # Simple status report

Function and object method calls

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

obj.some_method(x, y, z)

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

Variables and argument passing



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a = [1, 2, 3]



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b = a



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a.append(4)
b

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

In [27]: data = [1, 2, 3]

In [28]: append_element(data, 4)

In [29]: data
Out[29]: [1, 2, 3, 4]

Dynamic references, strong types



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



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'5' + 5



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a = 4.5
b = 2
# String formatting, to be visited later
print('a is {0}, b is {1}'.format(type(a), type(b)))
a / b



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a = 5
isinstance(a, int)



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

Attributes and methods

In [1]: a = 'foo'

In [2]: a.<Press Tab>
a.capitalize  a.format      a.isupper     a.rindex      a.strip
a.center      a.index       a.join        a.rjust       a.swapcase
a.count       a.isalnum     a.ljust       a.rpartition  a.title
a.decode      a.isalpha     a.lower       a.rsplit      a.translate
a.encode      a.isdigit     a.lstrip      a.rstrip      a.upper
a.endswith    a.islower     a.partition   a.split       a.zfill
a.expandtabs  a.isspace     a.replace     a.splitlines
a.find        a.istitle     a.rfind       a.startswith



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a = 'foo'



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getattr(a, 'split')

Duck typing



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



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

if not isinstance(x, list) and isiterable(x): x = list(x)

Imports

# 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)

Binary operators and comparisons



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5 - 7
12 + 21.5
5 <= 2



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a = [1, 2, 3]
b = a
c = list(a)
a is b
a is not c



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a == c



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a = None
a is None

Mutable and immutable objects



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



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

Scalar Types

Numeric types



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ival = 17239871
ival ** 6



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fval = 7.243
fval2 = 6.78e-5



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3 / 2



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3 // 2

Strings

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



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



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c.count('\n')



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



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a



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a = 5.6
s = str(a)
print(s)



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



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s = '12\\34'
print(s)



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



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



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template = '{0:.2f} {1:s} are worth US${2:d}'



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template.format(4.5560, 'Argentine Pesos', 1)

Bytes and Unicode



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val = "español"
val



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val_utf8 = val.encode('utf-8')
val_utf8
type(val_utf8)



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val_utf8.decode('utf-8')



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val.encode('latin1')
val.encode('utf-16')
val.encode('utf-16le')



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bytes_val = b'this is bytes'
bytes_val
decoded = bytes_val.decode('utf8')
decoded  # this is str (Unicode) now

Booleans



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True and True
False or True

Type casting



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

None



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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



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type(None)

Dates and times



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



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dt.date()
dt.time()



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



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



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



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



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dt
dt + delta

Control Flow

if, elif, and else

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 or equal to 5')



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



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4 > 3 > 2 > 1

for loops

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



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for i in range(4):
    for j in range(4):
        if j > i:
            break
        print((i, j))

for a, b, c in iterator:

# do something

while loops

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

pass

if x < 0: print('negative!') elif x == 0:

# TODO: put something smart here
pass

else: print('positive!')

range



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range(10)
list(range(10))



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list(range(0, 20, 2))
list(range(5, 0, -1))

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

sum = 0 for i in range(100000):

# % is the modulo operator
if i % 3 == 0 or i % 5 == 0:
    sum += i

Ternary expressions

value =



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