Python

High-level
General purpose
Multiple programming paradigms
Interpreted

Variables



In [1]:

    
var1 = 1              # interger
var2 = 2.34           # floating point numbers
var3 = 5.6 + 7.8j     # complex numbers 
var4 = "Hello World"  # strings
var5 = True           # booleans
var6 = None           # special value to indicate the absence of a value



In [2]:

    
print("var1 value:", var1, "type:", type(var1))
print("var2 value:", var2, "type:", type(var2))
print("var3 value:", var3, "type:", type(var3))
print("var4 value:", var4, "type:", type(var4))
print("var5 value:", var5, "type:", type(var5))
print("var6 value:", var6, "type:", type(var6))









    



var1 value: 1 type: <class 'int'>
var2 value: 2.34 type: <class 'float'>
var3 value: (5.6+7.8j) type: <class 'complex'>
var4 value: Hello World type: <class 'str'>
var5 value: True type: <class 'bool'>
var6 value: None type: <class 'NoneType'>

Containers

Data types for holding many variables

Lists

One-dimensional, ordered container whose contents and length can change (mutable).



In [3]:

    
hydrometeors = ['rain', 'snow', 'hail']  # create a list holding three elements
print(hydrometeors)
print('length:', len(hydrometeors))









    



['rain', 'snow', 'hail']
length: 3

The elements of a list do not need to be of the same type:



In [4]:

    
mixed_type_list = ['rain', 4.5, 99, None]
print(mixed_type_list)









    



['rain', 4.5, 99, None]

Elements can be added or removed from a list



In [5]:

    
hydrometeors = ['rain', 'snow', 'hail']
hydrometeors.append('drizzle')  # add 'drizzle' to the end of the list
print(hydrometeors)









    



['rain', 'snow', 'hail', 'drizzle']



In [6]:

    
hydrometeors = ['rain', 'snow', 'hail']
hydrometeors.insert(1, 'graupel')  # insert graupel before position 1
print(hydrometeors)









    



['rain', 'graupel', 'snow', 'hail']



In [7]:

    
hydrometeors = ['rain', 'snow', 'hail']
del hydrometeors[0]  # remove the first element from the list
print(hydrometeors)









    



['snow', 'hail']



In [8]:

    
hydrometeors = ['rain', 'snow', 'hail']
observation = hydrometeors.pop()  # remove the last item from the list and store it in hydrometeor
print("observation:", observation)
print("hydrometeors:", hydrometeors)









    



observation: hail
hydrometeors: ['rain', 'snow']

Elements of a list can be changed



In [9]:

    
hydrometeors = ['rain', 'snow', 'hail']
print("Before change:", hydrometeors)
hydrometeors[0] = 'virga'
print("After change:", hydrometeors)









    



Before change: ['rain', 'snow', 'hail']
After change: ['virga', 'snow', 'hail']

Lists Indexing



In [10]:

    
hydrometeors = ['rain', 'snow', 'hail']
print('index 0:', hydrometeors[0])   # indexing begins at 0
print('index 1:', hydrometeors[1])
print('index 2:', hydrometeors[2])









    



index 0: rain
index 1: snow
index 2: hail



In [11]:

    
hydrometeors[3]  # Trying to access elements which do not exist raises a IndexError









    



---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-11-7706668d582b> in <module>()
----> 1 hydrometeors[3]  # Trying to access elements which do not exist raises a IndexError

IndexError: list index out of range



In [12]:

    
hydrometeors = ['rain', 'snow', 'hail']
print('index -1:', hydrometeors[-1])
print('index -2:', hydrometeors[-2])
print('index -3:', hydrometeors[-3])









    



index -1: hail
index -2: snow
index -3: rain

List slicing

Syntax: list_variable[start:end:step]



In [13]:

    
hydrometeors = ['rain', 'snow', 'hail', 'drizzle', 'graupel', 'virga']
print(hydrometeors[2:4])  # select elements from index 2 to index 4









    



['hail', 'drizzle']



In [14]:

    
hydrometeors[:3]  # start from beginning









    Out[14]:





['rain', 'snow', 'hail']



In [15]:

    
hydrometeors[3:]  # until the end









    Out[15]:





['drizzle', 'graupel', 'virga']



In [16]:

    
hydrometeors[3:-1]  # negative indices









    Out[16]:





['drizzle', 'graupel']



In [17]:

    
hydrometeors[1::2]  # every 2nd element









    Out[17]:





['snow', 'drizzle', 'virga']

Tuples

One-dimensional, ordered container whose contents and length CANNOT change (immutable).



In [18]:

    
t = ('rain', 'snow', 'hail')
print(t)
print(len(t))









    



('rain', 'snow', 'hail')
3



In [19]:

    
t[0] = 'virga'  # tuples cannot be changed









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-19-d4fc2c3241e8> in <module>()
----> 1 t[0] = 'virga'  # tuples cannot be changed

TypeError: 'tuple' object does not support item assignment

Can be 'unpacked' to assign variable. Often used with functions which return multiple items.



In [20]:

    
observations = ('rain', 'snow', 'hail')  # tuple with three elements
obs1, obs2, obs3 = observations    # unpack tuple into obs1, obs2, obs3 variables
print("observations:", observations)
print("obs1:", obs1)
print("obs2:", obs2)
print("obs3:", obs3)









    



observations: ('rain', 'snow', 'hail')
obs1: rain
obs2: snow
obs3: hail

Dictionaries

Unordered collection of key/value pairs whose size and content can change



In [21]:

    
d = {'site': 'KLOT', 'amount': 20, 'wind': 'east'}



In [22]:

    
print(d.keys())









    



dict_keys(['wind', 'site', 'amount'])



In [23]:

    
print(d.values())









    



dict_values(['east', 'KLOT', 20])



In [24]:

    
print('site:', d['wind'])
print('amount:', d['amount'])
print('wind:', d['wind'])









    



site: east
amount: 20
wind: east



In [25]:

    
print("wind before change:", d['wind'])
d['wind'] = 'west'
print("wind after change:", d['wind'])









    



wind before change: east
wind after change: west

Entries can be added or remove from dictionaries



In [26]:

    
d = {'site': 'KLOT', 'amount': 20, 'wind': 'east'} 
print(d)









    



{'wind': 'east', 'site': 'KLOT', 'amount': 20}



In [27]:

    
del d['wind']
print(d)









    



{'site': 'KLOT', 'amount': 20}



In [28]:

    
d['wind_speed']  = 'east'
d['wind_direction'] = '10 m/s'
print(d)









    



{'wind_speed': 'east', 'wind_direction': '10 m/s', 'site': 'KLOT', 'amount': 20}

Note: Dictionaries do not preserve the order in which entries are added. If you need ordering use a OrderedDict from the collections module.

Flow control

If statements



In [29]:

    
hydrometeor = 'rain'
if hydrometeor == 'rain':
    print("You saw rain")









    



You saw rain



In [30]:

    
hydrometeor = 'hail'
if hydrometeor == 'rain':
    print("You saw rain")
else:
    print("You did NOT see rain")









    



You did NOT see rain



In [31]:

    
hydrometeor = 'snow'
if hydrometeor == 'rain':
    print("You saw rain")
elif hydrometeor == 'snow':
    print("You saw snow")
else:
    print("I do not know what you saw")









    



You saw snow

for loops

Syntax:
for variable in iterable:
code block



In [32]:

    
hydrometeors = ['rain', 'snow', 'hail']
for hydrometeor in hydrometeors:  # loop over elements in a list
    print(hydrometeor)









    



rain
snow
hail



In [33]:

    
for i in range(5):  # loop over the number 0 to 4
    print(i)



In [34]:

    
d = {'site': 'KLOT', 'amount': 20, 'wind': 'east'} 
for key, value in d.items():
    print(key, ':', value)









    



wind : east
site : KLOT
amount : 20

Functions



In [35]:

    
# simple
def func(arg1):
    print(arg1)
    return 42



In [36]:

    
# call a function
return_value = func("Hello World")
print("ret_value:", return_value)









    



Hello World
ret_value: 42

Functions can have multiple, no and even default arguments



In [37]:

    
def add_numbers(number1, number2):
    return number1 + number2

def say_hello():
    print("Hello AMS")
    
def favorite_hydrometeor(name, hydrometeor='snow'):
    print("Hello", name)
    print("Your favorite hydrometeor is", hydrometeor)



In [38]:

    
print(add_numbers(1, 2))



In [39]:

    
say_hello()









    



Hello AMS



In [40]:

    
favorite_hydrometeor("Jonathan")









    



Hello Jonathan
Your favorite hydrometeor is snow



In [41]:

    
favorite_hydrometeor("Jonathan", hydrometeor="hail")









    



Hello Jonathan
Your favorite hydrometeor is hail

Functions can return multiple values:



In [42]:

    
def sum_and_product(a, b):
    return a+b, a*b
sum_ab, product_ab = sum_and_product(2, 3)
print("sum", sum_ab)
print("product", product_ab)









    



sum 5
product 6

Classes



In [43]:

    
class Point(object):
    """ A class to store the coordinate in a plane"""
    
    def __init__(self, x, y):
        self.x = x   # an attribute
        self.y = y   # an attribute
        
    def sum_of_coordinates(self):  # a class method
        return self.x + self.y



In [44]:

    
home = Point(2, 3)



In [45]:

    
print(home.x)
print(home.y)

2
3



In [46]:

    
home.sum_of_coordinates()









    Out[46]:





5

Libraries

Python has a large number of libraries which extend the basic functionality.
The standard library is included with Python and contains a number of helpful features.
Third-party libraries can add even more powerful functionality!
Libraries must be imported to be used.



In [47]:

    
import math  # import the entire math module



In [48]:

    
math.sqrt(2)









    Out[48]:





1.4142135623730951



In [49]:

    
from random import randrange   # import just the randrange function from the random module



In [50]:

    
for i in range(5):
    print(randrange(1, 10))