Boston Python Workshop 3/Data types

Numbers: integers and floats

• Integers don't have a decimal place.
• Floats have a decimal place.
• Math mostly works the way it does on a calculator, and you can use parentheses to override the order of operations.

Math: addition, subtraction, multiplication

addition: 2 + 2
subtraction: 0 - 2
multiplication: 2 * 3

Math: division

>>> 4 / 2
2
>>> 1 / 2
0

• Integer division produces an integer. You need a number that knows about the decimal point to get a decimal out of division:

>>> 1.0 / 2
0.5
>>> float(1) / 2
0.5

Types

>>> type(1)
<type 'int'>
>>> type(1.0)
<type 'float'>

Strings

• Strings are bits of text, and contain characters like numbers, letters, whitespace, and punctuation.
• String are surrounded by quotes.
• Use triple-quotes (""") to create whitespace-preserving multi-line strings.

>>> "Hello"
'Hello'

String concatenation

>>> "Hello" + "World"
HelloWorld
>>> "Hello" + "World" + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects
>>> "Hello" + "World" + str(1)
'HelloWorld1'

Printing strings

>>> print "Hello" + "World"
HelloWorld
>>> print "Hello", "World"
Hello World
>>> print "Hello", "World", 1
Hello World 1

>>> print """In 2009,
...     The monetary component of the Nobel Prize
...         was US $1.4 million."""
In 2009,
    The monetary component of the Nobel Prize
        was US $1.4 million.

Types

>>> type("Hello")
<type 'str'>

Booleans

• There are two booleans, True and False.
• Use booleans to make decisions.

Containment with 'in' and 'not in'

>>> "H" in "Hello"
True
>>> "a" not in ["a", "b", "c"]
False

Equality

• == tests for equality
• != tests for inequality
• <, <=, >, and >= have the same meaning as in math class.

>>> 0 == 0
True
>>> 0 == 1
False

"a" != "a"

"a" != "A"

Use with if/else blocks

• When Python encounters the if keyword, it evaluates the expression following the keyword and before the colon. If that expression is True, Python executes the code in the indented code block under the if line. If that expression is False, Python skips over the code block.

temperature = 32
if temperature > 60 and temperature < 75:
    print "It's nice and cozy in here!"
else:
    print "Too extreme for me."

Types

>>> type(True)
<type 'bool'>
>>> type(False)
<type 'bool'>

Lists

• Use lists to store data where order matters.
• Lists are indexed starting with 0.

List initialization

>>> my_list = []
>>> my_list
[]
>>> your_list = ["a", "b", "c", 1, 2, 3]
>>> your_list
['a', 'b', 'c', 1, 2, 3]

Access and adding elements to a list

>>> len(my_list)
0
>>> my_list[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> my_list.append("Alice")
>>> my_list
['Alice']
>>> len(my_list)
1
>>> my_list[0]
'Alice'
>>> my_list.insert(0, "Amy")
>>> my_list
['Amy', 'Alice']

>>> my_list = ['Amy', 'Alice']
>>> 'Amy' in my_list
True
>>> 'Bob' in my_list
False

Changing elements in a list

>>> your_list = []
>>> your_list.append("apples")
>>> your_list[0]
'apples'
>>> your_list[0] = "bananas"
>>> your_list
['bananas']

Slicing lists

>>> her_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
>>> her_list[0]
'a'
>>> her_list[0:3]
['a', 'b', 'c']
>>> her_list[:3]
['a', 'b', 'c']
>>> her_list[-1]
'h'
>>> her_list[5:]
['f', 'g', 'h']
>>> her_list[:]
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

Sharing versus copying

Sharing

>>> my_list
['Alice']
>>> your_list = my_list
>>> your_list
['Alice']
>>> my_list[0] = "Bob"
>>> my_list
['Bob']
>>> your_list
['Bob']

Copying

>>> my_list
['Alice']
>>> your_list = my_list[:]
>>> my_list[0] = "Bob"
>>> my_list
['Bob']
>>> your_list
['Alice']

Strings are a lot like lists

>>> my_string = "Hello World"
>>> my_string[0]
'H'
>>> my_string[:5]
'Hello'
>>> my_string[6:]
'World'
>>> my_string = my_string[:6] + "Jessica"
>>> my_string
'Hello Jessica'

• One big way in which strings are different from lists is that lists are mutable (you can change them), and strings are immutable (you can't change them). To "change" a string you have to make a copy:

>>> h = "Hello"
>>> h[0] = "J"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> h = "J" + h[1:]
>>> h
'Jello'

Types

>>> type(my_list)
<type 'list'>

Dictionaries

• Use dictionaries to store key/value pairs.
• Dictionaries do not guarantee ordering.
• A given key can only have one value, but multiple keys can have the same value.

Initialization

>>> my_dict = {}
>>> my_dict
{}
>>> your_dict = {"Alice" : "chocolate", "Bob" : "strawberry", "Cara" : "mint chip"}
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}

Adding elements to a dictionary

>>> your_dict["Dora"] = "vanilla"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'chocolate'}

Accessing elements of a dictionary

>>> your_dict["Alice"]
'chocolate'
>>> your_dict.get("Alice")
'chocolate'

>>> your_dict["Eve"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Eve'
>>> "Eve" in her_dict
False
>>> "Alice" in her_dict
True
>>> your_dict.get("Eve")
>>> person = your_dict.get("Eve")
>>> print person
None
>>> print type(person)
<type 'NoneType'>
>>> your_dict.get("Alice")
'coconut'

Changing elements of a dictionary

>>> your_dict["Alice"] = "coconut"
>>> your_dict
{'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'coconut'}

Types

>>> type(my_dict)
<type 'dict'>