PyCon handout

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

Division
Integer division produces an integer:

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

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

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.

>>> print("Hello") 'Hello' >>> print("Python, I'm your #1 fan!") "Python, I'm your #1 fan!"

String Concatenation
You can smoosh strings together (called "concatenation") using the '+' sign:

>>> print("Hello" + "World") 'HelloWorld'

>>> name = "Jessica" >>> print("Hello " + name) 'Hello Jessica'

String length

 * Use the  function to get the length of a string
 * Use the  function to turn something that isn't a string into a string

>>> print(len("Hello")) 4 >>> print(len("")) 0 >>> fish = "humuhumunukunukuapuaʻa" >>> length = str(len(fish)) >>> print(fish + " is a Hawaiian fish whose name is " + length + " characters long.")

Quotes
You can surround a string with either double or single quotes. They mean the same thing:

>>> print('Hello') 'Hello' >>> print("Hello") 'Hello'

If your string contains a single quote as an apostrophe, surround the string in double quotes so Python isn't confused by the apostrophe:

>>> print("I'm a happy camper") "I'm a happy campter"

One fun thing about strings in Python is that you can multiply them:

>>> print("A" * 40) AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA >>> print "ABC" * 12 ABCABCABCABCABCABCABCABCABCABCABCABC >>> h = "Happy" >>> b = "Birthday" >>> print((h + b) * 10) HappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayHappyBirthday

Booleans

 * There are two booleans,  and.
 * Use booleans to make decisions.

Containment with 'in' and 'not in'
>>> "H" in "Hello" True >>> "H" not in "Hello" False >>> "Perl" not in "Boston Python Workshop" True

Equality

 * tests for equality
 * tests for inequality
 * ,,  , and   have the same meaning as in math class.

>>> 0 == 0 True >>> 0 == 1 False >>> "a" != "a" False >>> "a" != "A" True >>> 1 > 0 True >>> 2 >= 3 False >>> -1 < 0 True >>> .5 <= 1 True

'''Take note! '=' is for assignment and '==' is for comparison.'''

Flow Control

 * Use,   and   to make choices.
 * There is always exactly one ; it is always first and is testing some boolean condition.
 * You can have zero or many ;   also tests some boolean condition.
 * You can have zero or one ;   is a catch-all that goes at the end and does not depend on a boolean condition.

if statements
We can use these expressions that evaluate to booleans to make decisions and conditionally execute code:

>>> if "banana" in "bananarama": ...     print("I miss the 80s.") ... 'I miss the 80s.'

more choices: and
You can use the  keyword to execute code when the   expression isn't True. For example:

>>> sister_age = 15 >>> brother_age = 12 >>> if sister_age > brother_age: ...    print("sister is older") ... else: ...    print("brother is older") ... sister is older

Like with, the code block under the   statement must be indented so Python knows that it is a part of the   block.

compound conditionals: and

 * You can check multiple expressions together using the  and   keywords.
 * If two expressions are joined by an, they both have to be True for the overall expression to be True.
 * If two expressions are joined by an, as long as at least one is True, the overall expression is True.

>>> 1 > 0 and 1 < 2 True >>> 1 < 2 and "x" in "abc" False >>> "a" in "hello" or "e" in "hello" True >>> 1 <= 0 or "a" not in "abc" False

Here are examples of compound conditions in an  statement:

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

>>> hour = 11 >>> if hour 23: ...    print("Go away!") ...    print("I'm sleeping!") ... else: ...    print("Welcome to the cheese shop!") ...    print("Can I interest you in some choice gouda?") ... Welcome to the cheese shop! Can I interest you in some choice gouda?

You can have as many lines of code as you want in  and   blocks; just make sure to indent them so Python knows they are a part of the block.

even more choices:

 * If you have more than two cases, you can use the  keyword to check more cases.
 * You can have as many  cases as you want; Python will go down the code checking each   until it finds a True condition or reaches the default   block.

>>> sister_age = 15 >>> brother_age = 12 >>> if sister_age > brother_age: ...    print("sister is older") ... elif sister_age == brother_age: ...    print("sister and brother are the same age") ... else: ...    print("brother is older") ... sister is older

You don't have to have an  block, if you don't need it. That just means there isn't default code to execute when none of the  or   conditions are True.

Here's another example:

>>> color = "orange" >>> if color == "green" or color == "red": ...  print("Christmas color!") ... elif color == "black" or color == "orange": ...  print("Halloween color!") ... elif color == "pink": ...  print("Valentine's Day color!") ... Halloween color!

Remember that '=' is for assignment and '==' is for comparison.

In summary: the structure of if/elif/else
Here's a diagram of :



Lists

 * Use lists to store data where order matters.
 * Lists are indexed starting with 0.
 * Lists are denoted by comma-separated elements inside square brackets.

List initialization

 * An empty list is
 * Use the  function to get the length of a list

>>> my_list = [] >>> my_list [] >>> your_list = ["a", "b", "c", 1, 2, 3] >>> your_list ['a', 'b', 'c', 1, 2, 3] >>> len(your_list) 6

Access and adding elements to a list

 * Get individual elements from a list with [ ] syntax.
 * Use the  keyword to check if something is in a list.
 * Use negative numbers to get element from the end of a list.

>>> my_list = ['Amy', 'Alice', 'Adam'] >>> 'Amy' in my_list True >>> 'Bob' in my_list False >>> len(my_list) 3 >>> my_list[0] 'Amy' >>> my_list[1] 'Alice' >>> my_list[-1] 'Adam'

Changing elements in a list

 * Use  to add elements to the end of a list.
 * Use the [ ] syntax to change an element 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']

Strings are a lot like lists

 * You can use the [ ] and slicing syntax on strings, just like lists.

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

sorting lists
Use  to sort a list:

>>> names = ["Eliza", "Joe", "Henry", "Harriet", "Wanda", "Pat"] >>> names.sort >>> names ['Eliza', 'Harriet', 'Henry', 'Joe', 'Pat', 'Wanda']

Getting the maximum and minimum values from a list
>>> numbers = [0, 3, 10, -1] >>> max(numbers) 10 >>> min(numbers) -1

For loops

 * Use a  loop to do something for every element in a list.

>>> names = ["Jessica", "Adam", "Liz"] >>> for name in names: ...    print(name) ... Jessica Adam Liz

>>> names = ["Jessica", "Adam", "Liz"] >>> for name in names: ...    print("Hello " + name) ... Hello Jessica Hello Adam Hello Liz

statements inside loop

 * You can use  statements inside for loops

>>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]: ...    if name[0] in "AEIOU": ...        print(name + " starts with a vowel.") ... Alice starts with a vowel. Ellen starts with a vowel.

Sometimes you want to start with a new empty list, and only add to that list if some condition is true:

>>> vowel_names = [] >>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]: ...    if name[0] in "AEIOU": ...        vowel_names.append(name) ... >>> print(vowel_names) ['Alice', 'Ellen']

loops inside loops
You can put  loops inside   loops. The indentation dictates which  loop a line is in.

>>> letters = ["a", "b", "c"] >>> numbers = [1, 2, 3] >>> for letter in letters: ...    for number in numbers: ...        print(letter * number) ... a aa aaa b bb bbb c cc ccc

The order of the  loops matters. Compare the above example with this one:

>>> for number in numbers: ...    for letter in letters: ...        print(number * letter) ... a b c aa bb cc aaa bbb ccc