http://www.cs.cornell.edu/courses/cs1110/2019sp Lecture 3: Functions & Modules (Sections 3.1-3.3) CS 1110 Introduction to Computing Using Python [E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]
Function Calls • Function expressions have the form: fun (x,y,…) function argument name • Some math functions built into Python: >>> x = 5 >>> a = round(3.14159265) >>> y = 4 >>> a >>> bigger = max(x, y) 3 >>> bigger 5 Arguments can be any expression 2
Always-available Built-in Functions • You have seen many functions already § Type casting functions: int() , float() , bool() § Get type of a value: type() § Exit function: exit() Arguments go in () , but name() refers to function in general • Longer list: http://docs.python.org/3/library/functions.html 3
Modules • Many more functions available via built-in modules § “Libraries” of functions and variables • To access a module, use the import command: import < module name > Can then access functions like this: < module name > . < function name > ( < arguments > ) Example : >>> import math >>> p = math.ceil(3.14159265) >>> p 4 4
Module Variables • Modules can have variables, too • Can access them like this: < module name >.< variable name> • Example : >>> import math >>> math.pi 3.141592653589793 5
Visualizing functions & variables • So far just built-ins int() float() str() type() print() … C:\> python >>> 6
Visualizing functions & variables • So far just built-ins int() • Now we’ve defined a float() new variable str() type() print() … C:\> python x 7 >>> x = 7 >>> 7
Visualizing functions & variables • So far just built-ins int() • Now we’ve defined a float() new variable str() • Now we’ve imported a type() print() module … C:\> python x 7 math >>> x = 7 ceil() >>> import math sqrt() >>> e 2.718281 pi 3.14159 … 8
module help After importing a module, see what functions and variables are available: >>> help(< module name >) 9
Reading the Python Documentation https://docs.python.org/3/library/math.html 10
A Closer Reading of the Python Documentation https://docs.python.org/3/library/math.html Function name Possible arguments What the function Module evaluates to 11
Other Useful Modules • io § Read/write from files • random § Generate random numbers § Can pick any distribution • string § Useful string functions • sys § Information about your OS 12
Making your Own Module Write in a text editor We recommend Atom… …but any editor will work 13
Interactive Shell vs. Modules Python Interactive Shell Module • Written in text editor • Type python at command line • Loaded through import • Type commands after >>> • Python executes statements • Python executes as you type when import is called Section 2.4 in your textbook discusses a few differences 14
my_module.py Module Text Single line comment # my_module.py (not executed) """This is a simple module. Docstring It shows how modules work""" (note the Triple Quotes) Acts as a multi-line comment Useful for code documentation x = 1+2 Commands x = 3*x Executed on import 15
Modules Must be in Working Directory! Must run python from same folder as the module 16
Using a Module (my_module.py) Module Text Python Command Shell # my_module.py >>> import my_module Needs to be the same name """This is a simple module. as the file without the “.py” It shows how modules work""" x = 1+2 x = 3*x 17
On import…. Module Text Python Command Shell Python does not execute # my_module.py >>> import my_module (because of #) Python does not execute """This is a simple module. (because of """ and """) It shows how modules work""" my_module Python executes this. x = 1+2 x x = 3*x x 3 9 Python executes this. 18 variable x stays “within” the module
Clicker Question! Module Text Python Command Shell # my_module.py >>> import my_module After you hit “Return” here """This is a simple module. what will python print next? It shows how modules work""" (A) >>> (B) 9 x = 1+2 >>> x = 3*x (C) an error message (D) The text of my_module.py (E) Sorry, no clue. 19
Clicker Answer Module Text Python Command Shell # my_module.py >>> import my_module After you hit “Return” here """This is a simple module. what will python print next? It shows how modules work""" (A) >>> (B) 9 x = 1+2 >>> x = 3*x (C) an error message (D) The text of my_module.py (E) Sorry, no clue. 20
Using a Module (my_module.py) Module Text Python Command Shell # my_module.py >>> import my_module >>> my_module.x variable we want to access 9 """This is a simple module. It shows how modules work""" module name x = 1+2 my_module x = 3*x x 3 9 x 21
Windows command line You must import (Mac looks different) With import Without import C:\> python C:\> python >>> math.ceil(3.14159) >>> import math Traceback (most recent call last): >>> p = math.ceil(3.14159) File "<stdin>", line 1, in <module> >>> p NameError: name 'math' is not 4 defined Python unaware of math what “ math ” is p 4 ceil() sqrt() e 2.718281 pi 3.14159 … 22
You Must Use the Module Name >>> import my_module >>> import my_module >>> my_module.x >>> x Traceback (most recent call last): 9 File "<stdin>", line 1, in <module> NameError: name 'x' is not defined my_module my_module x x x 3 9 x 3 9 23
What does the docstring do? Module Text Python Command Shell # my_module.py """This is a simple module. It shows how modules work""" x = 1+2 x = 3*x 24
from command • You can also import like this: from <module> import <function name> • Example : >>> from math import pi no longer need the module name >>> pi 3.141592653589793 pi 3.141592653589793 25
from command • You can also import everything from a module : from <module> import * • Example : >>> from math import * ceil() >>> pi sqrt() e 3.141592653589793 2.718281828459045 pi 3.141592653589793 >>> ceil(pi) … 4 Module functions now behave like built-in functions 26
Dangers of Importing Everything >>> e = 12345 >>> from math import * >>> e 2.718281828459045 e 12345 2.718281828459045 ceil() e was sqrt() overwritten! pi 3.141592653589793 … 27
Avoiding from Keeps Variables Separate >>> e = 12345 >>> import math >>> math.e e 12345 2.718281828459045 math >>> e ceil() 12345 sqrt() e 2.718281 pi 3.14159 … 28
Ways of Executing Python Code 1. running the Python Interactive Shell 2. importing a module 3. NEW: running a script 29
Running a Script • From the command line, type: python <script filename> • Example: C:\> python my_module.py C:\> looks like nothing happened • Actually, something did happen § Python executed all of my_module.py 30
Running my_module.py as a script my_module.py Command Line Python does not execute (because of # ) # my_module.py C:\> python module.py Python does not execute """This is a simple module. (because of """ and """ ) It shows how modules work""" x Python executes this. x 3 9 x = 1+2 x = 3*x Python executes this. 31
Running my_module.py as a script my_module.py Command Line # my_module.py C:\> python my_module.py C:\> """This is a simple my_module. when the script ends, all memory It shows how modules work""" used by my_module.py is deleted thus, all variables get deleted x = 1+2 (including x) x = 3*x so there is no evidence that the script ran 32
Clicker Question my_module.py Command Line # my_module.py C:\> python my_module.py C:\> my_module.x """This is a simple my_module. After you hit “Return” here It shows how modules work""" what will be printed next? (A) >>> x = 1+2 (B) 9 >>> x = 3*x (C) an error message (D) The text of my_module.py (E) Sorry, no clue. 33
Clicker Answer my_module.py Command Line # my_module.py C:\> python my_module.py C:\> my_module.x """This is a simple my_module. After you hit “Return” here It shows how modules work""" what will be printed next? (A) >>> x = 1+2 (B) 9 >>> x = 3*x (C) an error message (D) The text of my_module.py (E) Sorry, no clue. 34
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