from Tkinter import *
import math

# Define my functions

def calculateTraversalInKM():
	# This method does the calculations.
	# First, create a couple 'constants'
	CLARKE1866EQRADIUS = 6378206.4 # Clarke 1866 equatorial radius in meters
	METERSPERKM = 1000
	
	# Make sure input text exists, it's numeric, and it's in range
	try:
		latDegreesString = e.get() # get the user's data in the text widget
		# If latDegreesString can't be cast to a float, a ValueError will be
		# raised by the float() function
		latDegrees = float(latDegreesString)
		# Now let's be sure that the latitude value makes sense in terms of
		# earth geometry. If it's not, we raise a ValueError so it can be
		# handled cleanly as an exception.
		if (latDegrees < -90 or latDegrees > 90):
			raise ValueError
		latRadians = math.radians(latDegrees) # need radians for cos() below
	except ValueError:
		# Bad input--set focus to text field, select all text, and
		# write error message to the output label
		e.focus_set()
		e.selection_range(0, END)
		# Announce the error in the string associated with outputLabel
		v.set("Error: input " + latDegreesString + " is illegal")
		return # A return here just goes back to the message loop. That's
		       # good--we don't want to proceed any further with bad data.
		

	# If we got here then the input text is ok--do the calculation
	circumferenceMeters = 2.0 * math.pi * CLARKE1866EQRADIUS * math.cos(latRadians)
	circumferenceKm = circumferenceMeters / METERSPERKM
	# Print the answer in the string associated with outputLabel 
	v.set("At " + latDegreesString + " degrees, your traversal distance would be " + \
		str(int(circumferenceKm)) +" km") # don't print float precision for convenience

def calculateCallback():
	# This is the callback function for goBtn.
	# When goBtn is clicked, we end up here--just call the calculation function. It's a good
	# idea to keep your callback functions small and clear--call other functions from here 
	# that do your application's real work
	calculateTraversalInKM()

if __name__ == '__main__':
	
	# NOTE: Python does not have block-local scope, meaning that something defined
	# inside an if statement has the same scope as anything defined just outside of it.
	# In this particular case, variables defined below have 'global' or the widest possible
	# scope and are therefore visible to our function definitions above.

	# Everything below is only run once. Any other action this program takes is a direct 
	# consequence of the user clicking goBtn.

	master = Tk() # create the main, or root window

	inputPromptLabel = Label(master, text="Enter a latitude in degrees:") # create a static label
	inputPromptLabel.pack()                                               # place the label on master

	e = Entry(master) # create an editable text widget
	e.pack()          # place the text widget on master
	e.focus_set()     # put cursor in text widget automatically as a user convenience


	goBtn = Button(master, text="Go", command=calculateCallback) # set up the one and only button
	goBtn.pack()                                                 # place it on master

	v = StringVar() # Set up an interactive label by associating a StringVar object (v) with the Label
	                # attribute textvariable. When we change the contents of v, it will show up
	                # in outputLabel.
	v.set("Traverse in km will appear here") # put some initial text into v
	outputLabel = Label(master, textvariable=v) # create and place outputLabel
	outputLabel.pack()

	mainloop() # start the message loop (defined somewhere else in Tkinter--don't worry about it)