Global variables in Python aa

Question

Hi I'm making a program on python and I'm having trouble adding a global variable to my program so I'm just going to post my code and show you how I tried to do it.

So this is my class:

import globalvariables

class Bus :  

def __init__(self, Number, Capacity, Destination, Seats):
    self.Bus_Number = Number
    self.Bus_Capacity = Capacity
    self.Bus_Destination = Destination
    self.Seats_taken = Seats

def Book(self):
    self.Seats_taken = Seats + 1

def ShowBus(self):
    return (str(self.Bus_Number) + ", " + str(self.Bus_Capacity) + ", " + str(self.Bus_Destination) + ", " + str(self.Seats_taken))

and this is my module for global variables

Seats = 0

and this is what I'm trying to run:

import Transport
import globalvariables

Big_Red = Transport.Bus(1, 50, "NYC", 0)
Big_Red.Book()

print(Big_Red.ShowBus())

I'm getting this error:

Traceback (most recent call last):
  File "D:\Python\Assignment 3\Tester.py", line 5, in <module>
    Big_Red.Book()
  File "D:\Python\Assignment 3\Transport.py", line 14, in Book
    self.Seats_taken = Seats + 1
NameError: global name 'Seats' is not defined

Ashwini Chaudhary · Accepted Answer · 2013-07-08 23:21:52Z

1

The variable Seats is local to __init__ function and can't be accessed outside of it.

So,

self.Seats_taken = Seats + 1

Should be :

self.Seats_taken =  self.Seats_taken + 1

or :

self.Seats_taken += 1

Instead of using global variables inside class you should use class attributes:

class Bus :
    seats = 50  #shared across all instances
    def __init__(self):
        #code 
    def Book(self):
        self.Seats_taken = self.seats + 1

edited Jul 8, 2013 at 23:21

answered Jul 8, 2013 at 22:51

Ashwini Chaudhary

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3 Comments

Panthy Over a year ago

thanks brah but if i did want to make a module of global variables and use that in my bus class, how would i do it?

Ashwini Chaudhary Over a year ago

@Panthy In which file did you define Seats variable?

Ashwini Chaudhary Over a year ago

@Panthy If it's inside globalvariables then use globalvariables.Seats. You should better create a class attribute Seats.

Archit Jain · Accepted Answer · 2013-07-08 23:07:28Z

1

Globals should be avoided. In case you still want it to be :

def Book(self):
    self.Seats_taken = globalvariables.Seats + 1

answered Jul 8, 2013 at 23:07

Archit Jain

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Comments

9000 · Accepted Answer · 2013-07-08 23:21:36Z

When you import globalvariables, you gain access to names qualified by the module name: globalvariables.Seats. To import Seats into the namespace of another module, use from globalvariables import Seats. (In desperate cases, you can import all names from a module: from globalvariables import *, but usually you don't want this.)

When you define a function, it has its own local namespace. It includes all function's arguments.

Seats = 100

def foo(Seats):
  # returns the value of variable named Seats
  # defined within "def foo", *not* defined by "Seats = 100"
  return Seats

print foo(200) # prints 200
print foo() # fails because Seats are not set

To initialize a function parameter, use default value:

def foo(seats=0):

print foo() # prints 0
print foo(55) # prints 55

Also, global variables are evil. Global constants are good.

You want to use a global variable to track seats taken. You'll be much better off if you encapsulate it into a class that only allows reasonable access, does not allow to set the value arbitrarily, log access if needed, etc:

class SeatsDispenser(object):
  def __init__(self, initial_count):
    self.count = initial_count

  def allocate(self, number_of_seats):
    self.count -= number_of_seats
    if self.count < 0:
      raise ValueError("Overcommitted!")

  def seats_left(self):
    return self.number_of_seats

Naming your variables, classes, constants, and functions with the same Title Case is impractical. Usually variables are lower_case, functions are lowerCamelCase, classes are TitleCamelCase and constants are ALL_CAPS.

A reasonable piece of code would look like this:

import constants # modules are usually lower case
import transport

def Bus(object):
  def __init__(self, number, capacity, seats=constants.SEATS):
    self.number = number
    self.capacity = capacity
    self.seats = seats

big_red = Bus(constants.NYC_BUS_NUMBER, 50, 25)
default_blue = Bus(1, 20) # seats not passed, the default value is used

seats_dispenser = SeatsDispenser(100)
seats_dispenser.allocate(big_red.count)
seats_dispenser.allocate(default_blue.count)
print seats_dispenser.seats.left()

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