🐍 Python Intermediate

🐍 Python Intermediate#

Purpose#

This tutorial introduces Pythonic coding practices and key Python data containers such as strings, lists, tuples, and dictionaries.

Reference: D. Beazley, Python Essential Reference, 4th ed, Addison Wesley, 2009

String Formatting#

Python offers multiple ways to format strings. Here are some common methods:

title = "The legendary pirate captain"
age = 726.25

# method 1: (formatted-string)%(values)
print("%s is %f years old."%(title, age))     # print a string and a floating point
print("%s is %.4f years old."%(title, age))   # 4 decimal places
print("%s is %10.4f years old."%(title, age)) # allocate 10 spaces
print("%s is %d years old."%(title, age))     # print it as integer
print("%s is %10d years old."%(title, age))   # allocate 10 spaces

# method 2: (formatted-string).format(values)
print("{} is {} years old.".format(title, age)) 
print("{} is {:0.4f} years old.".format(title, age))
print("{} is {:10.4f} years old.".format(title, age))

# method 3: f(formatted-string with values)
print(f"{title} is {age} years old.")
print(f"{title} is {age:.4f} years old.")
print(f"{title} is {age:10.4f} years old.")

Output:

The legendary pirate captain is 726.250000 years old.
The legendary pirate captain is 726.2500 years old.
The legendary pirate captain is   726.2500 years old.
The legendary pirate captain is 726 years old.
The legendary pirate captain is        726 years old.
The legendary pirate captain is 726.25 years old.
The legendary pirate captain is 726.2500 years old.
The legendary pirate captain is   726.2500 years old.
The legendary pirate captain is 726.25 years old.
The legendary pirate captain is 726.2500 years old.
The legendary pirate captain is   726.2500 years old.

Pythonic Coding#

"Pythonic" refers to writing code in a way that is idiomatic and aligns with the principles and style of the Python programming language. It emphasizes readability, simplicity, and elegance, often leveraging Python’s unique features and conventions.

Example: List Comprehensions#

Non-Pythonic:

squares = []
for i in range(10):
    squares.append(i * i)

Pythonic:
```
squares = [i * i for i in range(10)]
```

Simplifying Conditional Statements#

Consider the following code:

hand = input("What would you like to play? ")
if hand == "Rock":
    print("It is a valid play.")
elif hand == "Paper":
    print("It is a valid play.")
elif hand == "Sciccors":
    print("It is a valid play.")
else:    
    print("It is an invalid play.")

This code can be simplified by

hand = input("What would you like to play? ")
if hand == "Rock" or hand == "Paper" or hand == "Sciccors":
    print("It is a valid play")
else:    
    print("It is an invalid play")

This code can be further simplified using Python’s in keyword:

hand = input("What would you like to play? ")  # This is Pythonic
if hand in  ("Rock", "Paper", "Scissors"):  
    print("It is a valid play")
else:    
    print("It is an invalid play")    

Ternary Operator#

The ternary operator allows concise conditional assignments:

Consider the following code:

y = 1

if y == 1:
    x = 1
else: 
    x = 0
print(x)

The code can be simplified using Pythonic by

x = 1 if y == 1 else 0  # This is Pythonic
print(x)

Loops#

If you would like to use index in a loop:

# indexing using for loops
names = ['Peter Parker', 'Clark Kent', 'Wade Wilson', 'Bruce Wayne', 'Dr. Baek']
index = 0
for name in names:
    print(index, name)
    index += 1

This will print

Peter Parker
Clark Kent
Wade Wilson
Bruce Wayne
Dr. Baek

You can also use enumerate() for more Pythonic code:

names = ['Peter Parker', 'Clark Kent', 'Wade Wilson', 'Bruce Wayne', 'Dr. Baek']
for index, name in enumerate(names):
    print(index, name)

This will print the same.

You can also start the index from 1:

names = ['Peter Parker', 'Clark Kent', 'Wade Wilson', 'Bruce Wayne', 'Dr. Baek']
for index, name in enumerate(names, start=1):
    print(index, name)

This will print

Peter Parker
Clark Kent
Wade Wilson
Bruce Wayne
Dr. Baek

Iterating Through Multiple Lists#

You can iterate through multiple lists:

heroes = ['Spiderman', 'Superman', 'Deadpool', 'Batman', 'Pirate Captain']
for index, name in enumerate(names):
    hero = heroes[index]
    print(f'{name} is actually {hero}')

It will print

Peter Parker is actually Spiderman
Clark Kent is actually Superman
Wade Wilson is actually Deadpool
Bruce Wayne is actually Batman
Dr. Baek is actually Pirate Captain

You can the Pythonic style to iterate through multiple lists using zip():

# Pythonic with zip
for name, hero in zip(names, heroes):
    print(f'{name} is actually {hero}')

This will print the same.

Another example:

universes = ['Marvel', 'DC', 'Marvel', 'DC', 'USAFA']
for name, hero, universe in zip(names, heroes, universes):
    print(f'{name} is actually {hero} from {universe}')

This will print

Peter Parker is actually Spiderman from Marvel
Clark Kent is actually Superman from DC
Wade Wilson is actually Deadpool from Marvel
Bruce Wayne is actually Batman from DC
Dr. Baek is actually Pirate Captain from USAFA

Python Data Containers#

Strings#

Strings are created by enclosing characters in single, double, or triple quotes:

a = "Hellow World"
b = 'Python is groovy'
c = """Computer says 'No'"""
print(a, b, c)

The same type of quote used to start a string must be used to terminate it. Triple-quoted strings capture all the text that appears prior to the terminating triple quote, as opposed to single- and double-quoted strings, which must be specified on one logical line. Triple-quoted strings are useful when the contents of a string literal span multiple lines of text such as the following:
```
print('''Content-type: text/html

<h1> Hello World </h1>
Click <a href="http://www.python.org">here</a>.
''')
```

Strings are stored as sequences of characters indexed by integers starting at zero

a = "Hello World"
print(a[0])   # First character
print(a[:5])  # Slice: 'Hello'  
print(a[6:])  # Slice: 'World'
print(a[3:8]) # Slice: 'lo Wo' 

Try this:

x = "37"
y = "42"
z = x+y
print(z) # what does this print?  why?
z = int(x)+int(y)
print(z)  # what does this print?

Lists#

Lists are mutable sequences of objects, created with square brackets:
```
names = ["Dave", "Mark", "Ann", "Phil"]
print(names)
```
Lists are indexed by integers, starting with zero. Use the indexing operator to access and modify individual items of the list:
```
a = names[2]
print(a)
names[0] = "Jeff"
names.append("Paula")
names.insert(2, "Thomas")
print(names)
```
Lists can be sliced and concatenated using:
```
x[start:stop:stride]
```

For example,

b = names[0:2]
print(b)
c = names[2:]
print(c)
names[1] = 'Jeff'
print(names)
names[0:2] = ['Dave', 'Mark', 'Jeff']
print(names)

Use the plus + operator to concatenate lists:
```
a = [1,2,3]+[4,5,6]
print(a)
```

An empty list can be created by one of two ways

names = []
print(names)
names = list()
print(names)

Lists can contain any kind of Python object, including other lists, as in the following example:
```
a = [1, "Dave", 3.14, ["Mark", 7, 9, [0, 101]], 10]
```
Items contained in nested lists are accessed by applying more than one indexing operation, as follows:
```
print(a[1])
print(a[3])
print(a[3][1])
print(a[3][3][1])
```

Objects can be inserted and appended to a list.

# List methods (insert)
list_b.insert(2,'python')
print(list_b)
# List methods (append)
list_b.append('.')
print(list_b)

Tuples#

To create simple data structures, you can pack a collection of values together into a single object using a tuple. You can create a tuple by enclosing group of values in parentheses like this:
```
stock = ("GOOG", 100, 490.10)
address = ('www.python.org', 80)
person = ("Stan", "Baek", "stanley.baek@afacademy")
```

Python often recognizes that a tuple is intended even if the parentheses are missing

stock = "GOOG", 100, 490.10
address = 'www.python.org', 80
person = "Stan", "Baek", "stanley.baek@afacademy"

For completeness, 0- and 1-element tuples can be defined, but have special syntax:

a = ()    # 0-tuple, empty tuple
b = (1,)  # 1-tuple  
c = 1,    # 1-tuple
print(type(a))
print(type(b))
print(type(c))
b = (1)   # not a tuple
c = 1     # not a tuple
print(type(b))
print(type(c))

The values in a tuple can be extracted by numerical index just like a list. However, it is more common to unpack tuples into a set of variables like this
```
name, shares, price = stock
host, port = address
first_name, last_name, phone = person
```
Although tuples support most of the same operations as lists, such as indexing, slicing, and concatenation, the contents of a tuple cannot be modified after creation. That is, you cannot replace, delete, or append new elements to an exisiting tuple.
```
print(stock[1]) 
stock[1] = 200  # error
```

Note

Python tuples and strings are immutable. This means that once it is created, its contents cannot be changed — elements cannot be added, removed, or altered. On the other hand, Python lists are mutable.

Dictionaries#

In Python, a dictionary (or dict) is mapping between a set of indices (keys) and a set of values
The items in a dictionary are key-value pairs
Keys can be any Python data type
Dictionaries are unordered

We can create a dictionary using curly braces:

eng2sp = {'one': 'uno', 'two': 'dos', 'three': 'tres'}
print(eng2sp['one'])  # Output: uno

An empty dictionary can be created:

# Dictionaries
eng2sp = {}
eng2sp['one'] = 'uno'
print(eng2sp)

Keys can be retrieved using .keys() and values using .values():

# keys vs values
print(eng2sp.keys())
print(eng2sp.values())

🐍 Python Intermediate

Contents

🐍 Python Intermediate#

Purpose#

String Formatting#

Pythonic Coding#

Example: List Comprehensions#

Simplifying Conditional Statements#

Ternary Operator#

Loops#

Iterating Through Multiple Lists#

Python Data Containers#

Strings#

Lists#

Tuples#

Dictionaries#