Beginning Programming in Python

Fall 2019

Agenda

  • Modules:
    • Some useful modules
    • Hierarchical namespaces
    • Making your own modules
  • The main() function
  • PEP 8 (v. briefly)
  • A revisit to debugging, now that we're writing longer programs:
    • Looking at different error types (syntax, runtime, logical)

Modules

  • A module is a file that contains Python code
  • That file may contain functions that can be imported for your use
  • In general, Python standard library provides loads of useful modules for all sorts of things
# From a user perspective, modules are variables, functions, objects etc. 

import math 
# This line "imports" the math module, so that we can refer to it

math.log10(100) 
# Now we're calling a function from the math module to compute log_10(100)

Random module

Namespace & dot notation

  • To recap, the namespace is all identifiers available to a line of code

  • Namespaces are organized hierarchically into sub-pieces using modules, functions, and classes

  • If you want to use a function from another module you need to import it into the namespace of your code and use '.' notation

import math # Imports the math module into the current namespace

# The '.' syntax is a way of indicating membership
math.sqrt(2) # sqrt is a function that "belongs" to the math module

# (Later we'll see this notation reused with objects)

revisiting "import"

  • If you want to use a function in a module, then that module/function must be imported first
from math import sqrt

sqrt(2.0) # Now sqrt is a just a function in the current program's name space, 
# no dot notation required
from math import * # Import all functions from math

# But, this is generally a BAD IDEA, because you need to be  sure
# this doesn't bring in things that will collide with other things
# used by the program

log(10)
# More useful is the "as" modifier

from math import sqrt as square_root # This imports the sqrt function from math
# but names it square_root. This is useful if you want to abbreviate a long function
# name, or if you want to import two separate things with the same name

square_root(2.0)

what's in a module?

  • If a module is a part of your Python installation, then you can find out what it contains like this:

 

 

 

  • But if a module is "famous" (like all the ones we will be using), then just Google it!
help('random')

writing your own module

  • You can write your own modules:

    • Create a file whose name is x.py, where x is the name of the module you want to create

    • Edit x.py to contain the functions you want

    • Create a new file y.py in the same directory and include "import x" at the top of y.py

The main function

  • You may write a program and then want to reuse some of the functions by importing them into another program.

  • The problem is that when you import a module it is executed.

  • Question: How do you stop the original program from running when you import it as a module?

  • Answer: By putting the logic for the program in a "main()", which is only called if the program is being run by the user, not imported as a module.

Main function

5 minutes break!

pep8: use style

0
 Advanced issues found
▲
 

  • It is easy to rush and write poorly-structured, hard-to-read code. However, generally, this proves a false-economy, resulting in longer debug cycles, a larger maintenance burden, and less code reuse.

  • Although many sins have nothing to do with the cosmetics of the code, some can be fixed by adopting a consistent, sane set of coding conventions. Python did this with Python Enhancement Proposal (PEP) 8

pep8: use style

0
 Advanced issues found
▲
 
  • use 4 spaces (instead of tabs) for indentation
  • limit line length to 78 characters
  • when naming identifiers, use CamelCase for classes and lowercase_with_underscores for functions & variables
  • place imports at the top of the file
  • keep function definitions together
  • use docstrings to document functions
  • use two blank lines to separate function definitions from each other
  • keep top-level statements, including function calls, together at the bottom of the program

Debugging revisited

  • Syntax Error
  • Runtime Error
  • Semantic/Logical Error

Syntax error Example

runtime/semantic error

debug strategies

  • Use print statements dotted around the code to figure out what code is doing at specific points of time

  • Use a debugger - this allows you to step through execution, line-by-line, seeing what the program is up to at each step.

  • Write unit-tests for individual parts of the code

  • Use assert to check that expected properties are true

  • STARE HARD AT IT! Semantic errors will generally require you to question your program's logic.

debug Example

Lecture 10 challenge

Questions?

CSE 20 - Lecture 10

By Narges Norouzi

CSE 20 - Lecture 10

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