Python notes from codecademy. Intro: Python can be used to create web apps, games, even a search engine.

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1 Python notes from codecademy Chap 1 Python Syntax (13 ) Intro: Python can be used to create web apps, games, even a search engine. 1.1 Variables and Data Types Welcome print Welcome to Python! Variables Using variables store and work with different types of data. A variable stores a piece of data, and gives it a specific name. spam = 5 my_variable = 10 Booleans Numbers are one data type. A second data type is called a boolean. A Boolean is like a light switch, can only be True or False. a = True b = False <scrip.py> # set the variables to the values my_int = 7 my_float = 1.23 my_bool = True Reassign variables Change a variable by reassigning it. my_int = 3

2 # reassigns the value of variables my_int = 7 my_int = 3 print my_int Whitespace In Python, whitespace is used to structure code. Whitespace is important, so be careful to use it. # (Error message) Indentation Error: expected an indented block def spam() eggs = 12 return eggs print spam() Whitespace Means Right Space You ll get this error message Indentation Error: expected an indented block whenever your whitespace is off. The script will work in below format: def spam() eggs = 12 return eggs print spam() # outputs 12 A Matter of Interpretation The interpreter runs your code line by line, and checks for any errors. # creates two boolean variables

3 spam = True eggs = False Single Line Comments Use # sign for comments. A comment is a line of text that Python won t try to run as code and just for humans to read. Comments make your program easier to understand the code. Multi-Line Comments The # sign will only comment out a single line. Include the whole code block in a set of triple quotation marks (""" ) for multi-line comments. """ Comments more than one line exist in your codes""" Math Do some math in Python, such as add, subtract, multiply, divide numbers. addition = subtraction = multiplication = 108 * 0.5 division = 108/9 count_to = print count_to # outputs 2310 Exponentiation You can combine math with other data types (e.g. Booleans) and commands to create useful programs. Exponents syntax: eight = 2**3 # ** represents (2^3)

4 eggs = 10**2 eggs_2 = 10**10 print eggs, eggs_2 # outputs Modulo Another operator is modulo. Modulo returns the remainder from a division. e.g. 3 % 2 = 1 (2 goes into 3 evenly once, with 1 left over) spam = 7 % 6 print spam Bringing It All Together 1. Variables, which store values for later use 2. Data types, such as numbers and Booleans 3. Whitespace, which separates statements 4. Comments, which make your code easier to read 5. Arithmetic operations, including +, -, *, /, **, and % # creates variables and does some calculation on them monty = Ture python = monty_python = python ** 2 # outputs python squared Chap 1 Exercise: Tip Calculator (5 ) # declares variables meal = tax = tip = 0.15 # reassigns the value of meal variable

5 meal = meal + meal * tax # create a new variable to calculate the total amount total = meal + meal * tip # prints out in the format print("%.2f" % total) # outputs Chap 2 Strings & Console Output Strings String is another data type. A string can contain letters, numbers, and symbols. Strings need to be within quotes. name = Yingxie age = 23 food = cheese cake Escaping characters There s a snake in my boot! The code above runs in error because Python thinks the apostrophe in There s ends the string. Use backlash to fix it: There\ s a snake in my boot! Access by Index Each character in a string is assigned a number. The number is called index. In Python, start counting the index from zero instead of one. # the character at index zero of the string cats c = cats [0] # the character at index three of the string Ryan n = Ryan [3]

6 String methods Change strings using string methods perform specific tasks for strings. 1. len() the number of strings 2. lower() get rid of all the capitalization in strings 3. upper() make strings completely upper case 4. str() turns non-strings into strings parrot = Norwegian Blue print len(parrot) # outputs 14 parrot.lower() # outputs norwegian blue Ryan.lower() # outputs ryan parrot.upper() # outputs NORWEGIAN BLUE pi = 3.14 print str(3.14) # change the data type of pi from float to string Dot Notation len(string), str(object), but String.upper() and String.lower() Methods that use dot notation (.) only work with strings. len() and str() can work on other data types. lion = roar print len(lion) print lion.upper()

7 Printing Strings Our codes are written in editor, while the results of the code are shown in the console. print displays codes in the console print Monty Python Printing Variables the_machine_goes = Ping! print the_machine_goes String Concatenation Combine arithmetic operators and strings for concatenating a few strings together. print Life + of + Brian # outputs Life of Brian The operator between strings will add strings together, one after the other. Notice that there are spaces inside the quotation marks after Life and of so that can make the combined string look like 3 words. Explicit String Conversion Combine a string with sth. that isn t a string. Have to convert the non-string into a string. str() method achieves this goal. print I have + str(2) + coconuts! # outputs I have 2 coconuts! String Formatting with %, Part 1 A better method than concatenating strings together. The % operator after a string is used to combine a string with variables. name = Mike print Hello %s % (name) The % operator will replace a %s in the string with the string variable that comes after it

8 string_1 = Camelot string_2 = place print Let s not go to %s. Tis a silly %s. % (string_1, string_2) # outputs Lets not go to Camelot. Tis a silly place. String Formatting with %, Part 2 Use the % operator to replace the %s placehoders with the variables in paremtheses. Need the same number of %s terms in a string as the number of variables in parentheses: print The %s who %s %s! % ( Knights, say, Ni ) # outputs The Knights who say Ni! name = raw_input( What is your name? ) quest = raw_input( What is your quest? ) color = raw_input( What is your favorite color? ) print Ah, so your name is %s, your quest is %s, \ and your favorite color is %s % (name, quest, color) # outputs Ah, so your name is Yingxie, your quest is learning Python, and your favorite color is baby blue. Recap 1. Three ways to create strings Alpha (apostrophe) Bravo (quotes) str(3) (convert into string)

9 2. String methods len( Charlie ) Delta.upper() Echo.lower() 3. Printing a string print Foxtrot 4. Advanced printing techniques g = Golf h = Hotel print %s, %s % (g, h) Chap 3 Date and Time Keep track of when sth. happened. In Python use datetime to print the date and time in a nice format. The datetime Library # import library Getting the Current Date and Time Use datetime.now() function to retrieve the current date and time. print datetime.now() # print out the current date and time

10 print now # outputs :42: Extracting Information Don t want the entire date and time. current_year = now.year current_month = now.month current_day = now.day print now.year print now.month print now.day Hot Date Print today s date in a format, like mm/dd/yyyy. Use string substitution. print %s-%s-%s % (now.year, now.month, now.day) # outputs print '%s/%s/%s' % (now.month, now.day, now.year)

11 # outputs 2/26/2015 Pretty Time Do the same for the hour, minute, and second in the form of hh:mm:ss. print %s:%s:%s % (now.hour, now.minute, now.second) # outputs 10:58:30 Grand Finale print %s/%s/%s %s:%s:%s % (now.month, now.day, now.year, now.hour, now.minute, now.second) # outputs 2/26/ :03:26