Python data types cheat sheet
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
>>> name = "Jessica" >>> print("Hello " + name) Hello Jessica
>>> 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
andFalse
. - 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 isTrue
, Python executes the code in the indented code block under the if line. If that expression isFalse
, 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']
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 your_dict False >>> "Alice" in your_dict True >>> your_dict.get("Eve") >>> person = your_dict.get("Eve") >>> print(person) None >>> print(type(person)) <type 'NoneType'> >>> your_dict.get("Alice") 'chocolate'
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'>