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Python data types cheat sheet
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==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<br /> '''subtraction''': 0 - 2<br /> '''multiplication''': 2 * 3<br /> ====Math: division==== >>> 4 / 2 2.0 >>> 1 / 2 0.5 Division always produces a float (i.e., a number that knows about the decimal place). ====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, <code>True</code> and <code>False</code>. * Use booleans to make decisions. ====Containment with 'in' and 'not in'==== >>> "H" in "Hello" True >>> "a" not in ["a", "b", "c"] False ====Equality==== * <code>==</code> tests for equality * <codE>!=</code> tests for inequality * <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code> 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 <code>if</code> keyword, it evaluates the expression following the keyword and before the colon. If that expression is <code>True</code>, Python executes the code in the indented code block under the if line. If that expression is <code>False</code>, 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'> [[Category:CDSW]]
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