Python usage of commas [duplicate]

Question

In a forum I have found this nice function (done by Pixie) converting from Roman to Arabic numbers.

def decoder(r):
    k=r
    if r=="":return "Don't leave the input blank"
    roman,s= {"M":1000,"CM":900, "D":500, "CD":400, "C":100, "XC":90, "L":50, "XL":40, "X":10, "IX":9, "V":5, "IV":4, "I":1},0
    while r!="":
        if r[:2] in roman:a,r=r[:2],r[2:]
        elif r[0] in roman:a,r=r[0],r[1:]
        else: return "Enter proper Decimal/Roman number as input"
        s+=roman[a]
    return s if encoder(int(s))==k else "Not a valid Roman Numeral"


a="MCM"
print(decoder (a.upper))

I am a super newbie of Python, and I do not understand the statement

if r[:2] in roman:a,r=r[:2],r[2:]

I know r[:2] and others are string slicing. What I do not understand is the usage of the commas: a,r=r[:2],r[2:] looks as a tuple but why? Is it an assignment?

Please format your code by indenting it by 4 spaces. You can also highlight it and press ctrl+k when in the editor. — Carcigenicate
– Carcigenicate, Commented Jul 30, 2017 at 16:23

E. Ducateme · Accepted Answer · 2017-07-30 16:27:27Z

0

Python has the ability to assign to multiple values on the sample line...

In this case, I am also moving the assignment lines to a new, indented line under the if and elif statements.

if r[:2] in roman:
    a,r = r[:2], r[2:]

elif r[0] in roman:
    a,r = r[0], r[1:]

The first assignment line is the same as:

a = r[:2]
r = r[2:]

The second one is the same as:

a = r[0]
r = r[1:]

answered Jul 30, 2017 at 16:27

E. Ducateme

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2 Comments

tonypdmtr Over a year ago

Maybe in this case it happens to have the same effect. However, a,b=b,a is not the same as a=b followed by b=a

E. Ducateme Over a year ago

@tonypdmtr the code shown by the OP does not do what you are suggesting... for sake of argument, we could generalize r[:2] to be x and r[2:] to be y and thus a, r = x, y would be equivalent to a = x and r = y NOT the variable swapping exercise you suggest.

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