python.md

• ipython notebooks from DS4BME intro class
• python for R
• reticulate docs
• python tutorial

types

• types: type(10), type(10.0), type(10+0j), type(True)
• more complex data structures: type([1, 2]), type((1, 2)), type({1, 2})
• conversions: type(float(10)), a=10, type(a.__float__())

Getting started

• start up python with python3
• python --version
• quit()
• print("Hello World")
• Version 2.7 (on which a lot of software was built) is no longer supported
• Try some basic calculations
• Try word = "string test" then word[0], word[1], word[1 : 2]
• Try word2 = "string test2" then word + word
• Creating a list dat = [1, 4, 8, 10] try dat[0] , dat[-1], dat[2 : 4] dat[2 : ] dat[:2]
  • dat2 = [dat, dat] dat2[1][2]
  • dat3 = [dat2, "string1"]
  • dat4 = dat + dat
• Multiple assignments a, b = 2, [2, 4]
• 2 ** 3
• 4.1 / 2 4.1 // 2 4.1 % 2

Basic programming

• input for text input as in x = input(are you mean (y/n)? > `)

if x == 'y': 
 print("Slytherine!")
else:
 print("Gryffindor")

• Notice that indentation matters in python (instead of using curly braces, say), don't forget the :
• Control flow

if statement1 :
 ...
elif statement2 :
 ...
else 
 ...

• Example, note this is just (a < 0) - (a > 0)

a = 5

if a < 0 :
  a = -1
elif a > 0 :
  a = 1
else :
  a = 0

• For and while loops

for i in range(10) :
 print(i)
 
x = 10
while x > 0 :
 x = x - 1
 print(x)

• Note that for can loop over any indexed entity

for w in 'word':
 print(w)

weirdList = ["a", ["b", 2], 3]
for w in weirdList :
 print(w)

• The range function is useful for numerical indexing, try range(5)
• Creating functions

## Remember the colon
def pow(x, n = 2):
  return x ** n

pow(3, 2)
pow(x = 3, n = 2)
pow(n = 2, x = 3)
pow(n = 2, 3) 

• From https://docs.python.org/3/tutorial/controlflow.html control over whether an argument is positional, keyword or both. (Don't get too cute with this.)

def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
      -----------    ----------     ----------
        |             |                  |
        |        Positional or keyword   |
        |                                - Keyword only
         -- Positional only

• python has its version of ... (also from docs.python.org)

def concat(*args, sep="/"):
 return sep.join(args)  

concat("a", "b", "c")

• lambda is for creating short unnamed function definitions. Let's return to an example that we did in R

def makepow(n):
 return lambda x: x ** n

square = makepow(2)
square(3)
cube = makepow(3)
cube(2)

Data structures

• We've already seen lists. Python has other data structures built in.
• Sets {"a", "a", "a", "b"} (unique elements)
• Tuples (1, 2, 3) (a lot like lists but not mutable, i.e. need to create a new to modify)
• Dictionaries

dict = {"a" : 1, "b" : 2}
dict['a']
dict['b']

• More about data structures can be founds at the python docs

Modules and running code

• loading a model. Define a file mymod.py with the folllowing function defs

def square(x) :
  return x ** 2


def cube(x) :
  return x ** 3

• then

import mymod as mm
mm.square(2)
mm.cube(2)
from mymod import square
square(2)
dir(mm)

• Add print(square(2), cube(3)) to the end of mymod.py. How to run it from the command line?

1. python3 mymod.py
2. python3 < mymod.py
3. Add #!/usr/bin/python3 then chmod +x mymod.py then ./mymod.py