Introduction To Python¶

This is a collection of various statements, features, etc. of IPython and the Python language. Much of this content is taken from other notebooks so I can't take credit for it, I just extracted the highlights I felt were most useful.

Code cells are run by pressing shift-enter or using the play button in the toolbar.

In [1]:

a = 10

In [2]:

print(a)

In [3]:

import math

In [4]:

x = math.cos(2 * math.pi)
print(x)

1.0

Import the whole module into the current namespace instead.

In [5]:

from math import *
x = cos(2 * pi)
print(x)

1.0

Several ways to look at documentation for a module.

In [6]:

print(dir(math))

['__doc__', '__name__', '__package__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']

In [7]:

help(math.cos)

Help on built-in function cos in module math:

cos(...)
    cos(x)
    
    Return the cosine of x (measured in radians).

Variables¶

In [8]:

x = 1.0
type(x)

Out[8]:

float

In [9]:

# dynamically typed
x = 1
type(x)

Out[9]:

int

Operators¶

In [10]:

1 + 2, 1 - 2, 1 * 2, 1 / 2

Out[10]:

(3, -1, 2, 0)

In [11]:

# integer division of float numbers
3.0 // 2.0

Out[11]:

1.0

In [12]:

# power operator
2 ** 2

Out[12]:

In [13]:

True and False

Out[13]:

False

In [14]:

not False

Out[14]:

True

In [15]:

True or False

Out[15]:

True

In [16]:

2 > 1, 2 < 1, 2 > 2, 2 < 2, 2 >= 2, 2 <= 2

Out[16]:

(True, False, False, False, True, True)

In [17]:

# equality
[1,2] == [1,2]

Out[17]:

True

Strings¶

In [18]:

s = "Hello world"
type(s)

Out[18]:

str

In [19]:

len(s)

Out[19]:

In [20]:

s2 = s.replace("world", "test")
print(s2)

Hello test

In [21]:

s[0]

Out[21]:

'H'

In [22]:

s[0:5]

Out[22]:

'Hello'

In [23]:

s[6:]

Out[23]:

'world'

In [24]:

s[:]

Out[24]:

'Hello world'

In [25]:

# define step size of 2
s[::2]

Out[25]:

'Hlowrd'

In [26]:

# automatically adds a space
print("str1", "str2", "str3")

('str1', 'str2', 'str3')

In [27]:

# C-style formatting
print("value = %f" % 1.0)

value = 1.000000

In [28]:

# alternative, more intuitive way of formatting a string 
s3 = 'value1 = {0}, value2 = {1}'.format(3.1415, 1.5)
print(s3)

value1 = 3.1415, value2 = 1.5

Lists¶

In [29]:

l = [1,2,3,4]

print(type(l))
print(l)

<type 'list'>
[1, 2, 3, 4]

In [30]:

print(l[1:3])
print(l[::2])

[2, 3]
[1, 3]

In [31]:

l[0]

Out[31]:

In [32]:

# don't have to be the same type
l = [1, 'a', 1.0, 1-1j]
print(l)

[1, 'a', 1.0, (1-1j)]

In [33]:

start = 10
stop = 30
step = 2
range(start, stop, step)

# consume the iterator created by range
list(range(start, stop, step))

Out[33]:

[10, 12, 14, 16, 18, 20, 22, 24, 26, 28]

In [34]:

# create a new empty list
l = []

# add an elements using `append`
l.append("A")
l.append("d")
l.append("d")

print(l)

['A', 'd', 'd']

In [35]:

l[1:3] = ["b", "c"]
print(l)

['A', 'b', 'c']

In [36]:

l.insert(0, "i")
l.insert(1, "n")
l.insert(2, "s")
l.insert(3, "e")
l.insert(4, "r")
l.insert(5, "t")

print(l)

['i', 'n', 's', 'e', 'r', 't', 'A', 'b', 'c']

In [37]:

l.remove("A")
print(l)

['i', 'n', 's', 'e', 'r', 't', 'b', 'c']

In [38]:

del l[7]
del l[6]

print(l)

['i', 'n', 's', 'e', 'r', 't']

Tuples¶

In [39]:

point = (10, 20)
print(point, type(point))

((10, 20), <type 'tuple'>)

In [40]:

# unpacking
x, y = point

print("x =", x)
print("y =", y)

('x =', 10)
('y =', 20)

Dictionaries¶

In [41]:

params = {"parameter1" : 1.0,
          "parameter2" : 2.0,
          "parameter3" : 3.0,}

print(type(params))
print(params)

<type 'dict'>
{'parameter1': 1.0, 'parameter3': 3.0, 'parameter2': 2.0}

In [42]:

params["parameter1"] = "A"
params["parameter2"] = "B"

# add a new entry
params["parameter4"] = "D"

print("parameter1 = " + str(params["parameter1"]))
print("parameter2 = " + str(params["parameter2"]))
print("parameter3 = " + str(params["parameter3"]))
print("parameter4 = " + str(params["parameter4"]))

parameter1 = A
parameter2 = B
parameter3 = 3.0
parameter4 = D

Control Flow¶

In [43]:

statement1 = False
statement2 = False

if statement1:
    print("statement1 is True")
elif statement2:
    print("statement2 is True")
else:
    print("statement1 and statement2 are False")

statement1 and statement2 are False

Loops¶

In [44]:

for x in range(4):
    print(x)

In [45]:

for word in ["scientific", "computing", "with", "python"]:
    print(word)

scientific
computing
with
python

In [46]:

for key, value in params.items():
    print(key + " = " + str(value))

parameter4 = D
parameter1 = A
parameter3 = 3.0
parameter2 = B

In [47]:

for idx, x in enumerate(range(-3,3)):
    print(idx, x)

(0, -3)
(1, -2)
(2, -1)
(3, 0)
(4, 1)
(5, 2)

In [48]:

l1 = [x**2 for x in range(0,5)]
print(l1)

[0, 1, 4, 9, 16]

In [49]:

i = 0
while i < 5:
    print(i)
    i = i + 1
print("done")

0
1
2
3
4
done

Functions¶

In [50]:

# include a docstring
def func(s):
    """
    Print a string 's' and tell how many characters it has    
    """
    
    print(s + " has " + str(len(s)) + " characters")

In [51]:

help(func)

Help on function func in module __main__:

func(s)
    Print a string 's' and tell how many characters it has

In [52]:

func("test")

test has 4 characters

In [53]:

def square(x):
    return x ** 2

In [54]:

square(5)

Out[54]:

In [55]:

# multiple return values
def powers(x):
    return x ** 2, x ** 3, x ** 4

In [56]:

powers(5)

Out[56]:

(25, 125, 625)

In [57]:

x2, x3, x4 = powers(5)
print(x3)

In [58]:

f1 = lambda x: x**2
f1(5)

Out[58]:

In [59]:

map(lambda x: x**2, range(-3,4))

Out[59]:

[9, 4, 1, 0, 1, 4, 9]

In [60]:

# convert iterator to list
list(map(lambda x: x**2, range(-3,4)))

Out[60]:

[9, 4, 1, 0, 1, 4, 9]

Classes¶

In [61]:

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
        
    def translate(self, dx, dy):
        self.x += dx
        self.y += dy
        
    def __str__(self):
        return("Point at [%f, %f]" % (self.x, self.y))

In [62]:

p1 = Point(0, 0)
print(p1)

Point at [0.000000, 0.000000]

In [63]:

p2 = Point(1, 1)

p1.translate(0.25, 1.5)

print(p1)
print(p2)

Point at [0.250000, 1.500000]
Point at [1.000000, 1.000000]

Exceptions¶

In [64]:

try:
    print(test)
except:
    print("Caught an expection")

Caught an expection

In [65]:

try:
    print(test)
except Exception as e:
    print("Caught an exception: " + str(e))

Caught an exception: name 'test' is not defined