Python

python [filename] python3 [filename]

Standard Library

run if not modules

if __name__ == "__main__":

Exceptions

try:
    x = int(input("Please enter a number: "))
    break
except OSError as err:
    print("OS error: {0}".format(err))
except ValueError:
    print("Could not convert data to an integer.")
except:
    print("Unexpected error:", sys.exc_info()[0])
    raise #reraise error

Access Args

import sys
sys.argv #the list of command-line arguments with [0] being the file name

Can also use argparse

Only function scoping

if y > threshold:
  x = 1
print(x)

and and or

• 0, '', [], (), {}, and None are false

• Actually return one of the values being computed

  • 'a' and 'b' returns 'b'

  • '' and 'b' return ''

• So, 1 and a or b is basically the trinary operator(unless a is false)

• (1 and [a] or [b])[0] is the safe way

Math

round(5.0000, 3) #3 is number of decimal points

'' vs ""

They are the same

== vs is

is checks they point to the same thing i.e [1,2,3] is not [1,2,3] but [1,2,3] == [1,2,3]

Use is None because a class could define == to be different and is is faster

Checking Packages

dir(nltk) - lists all functions in package

Functions

KArgs & Args

*args will give you all function parameters as a tuple

**kwargs will give you all keyword arguments as a dictionary expect acutal args

Pass in dict like f(**dict) to act as **kwargs

Lambdas

lambda x, y: x[1] + y

Generators

def simpleGeneratorFun(): 
    yield 1
    yield 2
    yield 3
  
for value in simpleGeneratorFun():  
    print(value)

Input

• PYTHON 2: input("Enter number") interprets user input so if int, int will be returned(Security bug, runs arbitrary code use raw_input)

• raw_input("Enter Your Name: ") takes exactly what user typed

• PYTHON 3: input is raw_input

Exceptions

try:
    x = int(input("Please enter a number: "))
    raise ValueError("Hello")
except ValueError:
    print("Oops!  That was no valid number.  Try again...")

types

str(1) #'1'

Other

value_when_true if condition else value_when_false