Python: confused about assignment in nested lists

Question

I put a polynomial in a nested list; for example 5X^3+4X^2+8X. I stored coefficients (5,4,8) in first list and exponents(3,2,1) in second:

polynom = [[5,4,8],[3,2,0]]

Then I define a function to pop the last term of coefficients and exponents like this

def expon_coef_pop(seq):
 expon = seq[1]
 coef = seq[0]
 expon.pop()
 coef.pop()
 return coef, expon
print(expon_coef_pop(polynom))
print(polynom)
# polynom changed into [[5,4],[4,2]]

Surprisingly, I found the value of polynom turned into [[5,4],[3,2]]. I thought I just modified value of expon and coef.

I don't want to change the value of polynomial.

How could this happen, and how to deal with this problem? I am confused about why the polynorm changed not the function. (I just wrote the function for a simple example.)

• m0bi5: how is it 8X shouldnt it be 8 as X^0 is 1
• bereal: coeff and expon are references to the same list objects as seq[0] and seq[1].
• Sean Xie: Sorry, a mistake (●くろまる'◡'●くろまる)

Answer 1

Both coef and expon are references pointing at the same list objects as seq[0] and seq[1] correspondingly. You will need to copy the lists before popping from them, which can also be done all in one step:

def expon_coef_pop(seq):
 return seq[0][:-1], seq[1][:-1]

Answer 2

It's because you pass your lists by reference. Compare with this assignment with copying

expon = list(seq[1])
coef = list(seq[0])

• Sean Xie: That`s just what I need! Thank you!
• Sean Xie: it said 'TypeError: 'list' object is not callable'
• m0bi5: @SeanXie try my answer and let me know
• Yury: @SeanXie I checked it on both python 2 and python 3 and it works. Maybe you have object called 'list' in another part of your program? If it's true, just rename it.
• Sean Xie: I just tried expon=list(polynom[0]) in python 3.5 console. It still said TypeError

Answer 3

Just change simple modification. You can try it:

you need to Change

expon = seq[1] to expon = seq[1][:]
coef = seq[0] to coef = seq[0][:]

Your final code:

polynom = [[5,4,8],[3,2,0]]
def expon_coef_pop(seq):
 expon = seq[1][:]
 coef = seq[0][:]
 expon.pop()
 coef.pop()
 return coef, expon
print(expon_coef_pop(polynom))
print(polynom)