Useful py stuff I keep forgetting…

Get the time elapsed since a process started:

begin = datetime.datetime.now()
# do some stuff…
duration=datetime.datetime.now() – begin
print(“finished after “+str(duration.seconds)+” seconds”) 

Working with strings

• if ‘dog’ in ‘hotdog’ # returns True
• “hot$dog”.split(“$”) # returns the sequence ['hot','dog']

Formatting

• “value %i” % myValue
  Formats myValue as an integer
• “value “+str(5)
  Convert 5 to a string and concatenate it with “value “
  

Arrays (aka python sequences)

• nOObs=[] # create an empty array
• len(array) # returns the number of elements
• array.append(elem) # add an element

Math

• fabs(x) in math (absolute value)
• floor(x), ceil(x)
• division by zero yields an error unlike C/C++

Iterators

• for x in arr # if arr is an array/sequence
• for x in range(N) # iterate in the range [0,N[
• for x in range(1,N) # iterate in the range [1,N[

Importing modules

• from math import fabs
  => import only the fabs function
• from math import *
  => import everything from the math module
• import math
  => import the math module (still needs math.fabs(x) instead of fabs(x) )

Truths and Falsehoods

• True # gets an uppercase
• False # gets an uppercase
• if(None) # that is False
• if(None==False) # that is also False
• true, false, yes, no, Yes, No are undefined

Caveats

• if(x==y) return 0 # WRONG
  => if(x==y) : return  0 #OK
• if(!x) # WRONG
  => if not x
• (x||y) # WRONG
  => x or y
• (x && y) # WRONG
  => x and y
• def myFunction() # MISSING COLON AT END
  => def myFunction():
• “value: “+5 # WRONG
  => “value: ” + str(5)