Python Cheat Sheet

This is for everyone who, for whatever reason, are putting off learning Python.  Ideally, if you are coming from another language such as MEL or Perl, you should be able to skim through these demos and have no problem transitioning to Pythonic syntax.  The following Python snippets are selections from the online documentation at http://docs.python.org/, which I modified whenever possible to fit into a Maya context.

Data Structures

• Strings

myString="Hello"
mySecondString="World"
result=myString+mySecondString
myMultilineString="""Hello World.
What a beautiful world it is."""
myRawString=r"C:/Users"
  
firstLetter=myString[1]
lastLetter=myString[-1]
firstWord=myString[0:4] 

• Unicode: Since Maya is used internationally, multiple language packs are supported which require a unicode character set. Think of unicode as the big brother of the standard ASCII model. You will not see anything unusual as the result of MEL commands, however Maya will return Python commands in unicode notation. As long as your in Maya the prefix u can be ignored, however, if you are writing output to a file they will have to be converted to ASCII.
  

print cmds.ls(cameras=1)
>>[u'frontShape', u'perspShape', u'sideShape', u'topShape']

• Lists

print cmds.ls(cameras=1)
>>[u'frontShape', u'perspShape', u'sideShape', u'topShape']

myToys=["yoyo", "cup", "ball", "crayon"] #the names of your objects
cmds.select(myToys)

firstToy=myToys[1]  #yoyo
numToys=len(myToys)  #3

• List Comprehensions: These are used to create lists from any iterable object.  The first time I used them was to retrieve the nth column in a matrix stored as vectors.
  

myMatrix=[[1,2,3,4],[1,2,3,4],[1,2,3,4]]
thirdRow=myMatrix[2] #[1,2,3]
thirdColumn=[x[2] for x in myMatrix] #[3,3,3]

• Tuples: Are similar to list except they cannot be modified, also known as immutable.

myList=["Hello", 1]
myList[1]=5 #can change the value

myTuple=("Hello", 1) 
myTuple[1]=5 #ERROR

• Sets: Sets are GREAT.  They are just like lists but prevent duplicate elements.

keyLight= cmds.lightlink(light="keyLight", query=1) #return all objects set to receive light from keyLight
backLight= cmds.lightlink(light="backLight", query=1) #return all objects set to receive light from backLight

lightsList=keyLight+backLight #12 items, lists duplicates
lightsSet=set(keyLight+backLight) #8 items, returns only unique elements

• Dictionaries: Sometimes referred to as hashes or associative arrays, are used to store data in name, value pairs.

myCameras=cmds.ls(cameras=1)
myTextures=cmds.ls(textures=1)
myScene={'cameras':myCameras, 'textures':myTextures} #myScene stores all the cameras and textures in the scene

print myScene.keys()
print myScene.values()
print myScene["cameras"]

Control Flow

• While Loops: Use extreme caution when using while loops. There is no way to close Maya if you get caught in an infinite loop.
  

while x<10:
    print x
    x+=1 #make sure to terminate the loop!

• If Statement

if (myValue==10 or myValue==20) and myValue!=30
    print myValue
elif myValue==0:
    print "Cannot be 0"
else: #all occurances
    pass #pass keeps Pythonic syntax, useful as placeholders 

• For Statement

for x in range(10):
    print x, someList[x]

for index, eachItem in enumerate(iterableItem):
    print index, eachItem

• Exceptions

sphereAttrs=cmds.listAttr('pSphere1')
for eachAttr in sphereAttrs:
    print eachAttr+":",
    try:
        print cmds.getAttr("pSphere1."+eachAttr)
    except:
        print "Cannot Read"

Functions

def myFunction(arg=None): #defines function myFunction that accepts an argument arg that defaults to None
    print arg
    return 1
myFunction("Hello World") #calls myFunction

Classes

class newClass():
    myInt=5 #class variables
    @classmethod  #class method decorator
    def getInt(cls): #class method
        return cls.myInt
    def __init__(self, arg): #gets automatically called when an object is instantiated
        self.myString=arg  #self are instance variables, they are unique per object
    def myFunction(self, arg):
        print arg, self.myString

myClass =newClass("Hello") #creates a new object from class newClass, objects are instances of a class
myClass.myFunction("World") #Hello World
print myClass.myFunction.myString #Hello
print newClass.myInt #5
print newClass.getInt() #5

I hope this will get you all started in the right direction. The vast majority of all your scripts will come from topics discussed here, everything else beyond this cheat sheet will merit a dedicated post. If you have further questions I find it's easiest to just to Google the topic. Even though I am a member of http://www.python-forum.org/pythonforum/ I rarely, if ever have to create a new thread. On the other hand I highly recommend the Maya Programming Forum on http://forums.cgsociety.org/ since Maya specifics sometimes get hazy. Hopefully you are all already members.