Can assignment expression create Fibonacci series using list comprehension?

Question

With assignment expression, I thought I could try list comprehension to create Fibonacci. I first initialize a Fibonacci list of 5 elements f = [1,2,3,4,5] with the first two values being the seeds. The test run below shows the assignment expression works.

[y := f[n-1] + f[n-2] for n in range(2,6)] 
[3, 5, 7, 9]

But the real Fibonacci failed at f[n] where a red marker showed in Python shell.

[f[n] := f[n-1] + f[n-2] for n in range(2,6)]

Is it because the f[n] is not a valid variable name?
Does that mean assignment expression might not help in list comprehension for Fibonacci?

Assignment expressions were added recently in Python 3.8. Make sure the language level of your IDE is set to this. — Barmar, Oct 27 '20 at 05:45
Your fibonacci expression is not correct, it seems like you're only generating odd numbers. Try `s = [0, 1]; result = s + [(s := [s[1], s[0] + s[1]]) and s[1] for k in range(10)]` — cs95, Oct 27 '20 at 06:02
Did you look at the error message you got? It says "SyntaxError: cannot use assignment expressions with subscript". Thus, you cannot use assignment expressions to update an element of a list, as you try. — JohanL, Oct 27 '20 at 06:10
I would like to argue that this is **not** a duplicate as currently set, since this is a question about the assignment operator, rather than the Fibonacci sequence. The latter is more of an example. — JohanL, Oct 27 '20 at 06:13
Thanks for all the replies. As @cs95 suggested, `f = [0, 1]; f += [(f := [f[1], f[0] + f[1]]) and f[1] for n in range(10)]` works without needing f[n] :=. By the way, I am using the IDE PyCharm with Python3.8, When I hovered over the red marker, it showed 'identifier expected'. How do I get the error message "SyntaxError ..."? — Leon Chang, Oct 28 '20 at 03:53

martineau · Accepted Answer · 2020-10-28T08:22:55.170

1

Using [f[n] := ... results in SyntaxError: cannot use assignment expressions with subscript, so clearly this restriction prevents doing it that way. This is mentioned in PEP 572 in a section titled Differences between assignment expressions and assignment statements where is says

Single assignment targets other than a single NAME are not supported.

edited Oct 28 '20 at 08:22

answered Oct 27 '20 at 06:14

martineau

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score 0 · Answer 2 · answered Nov 06 '20 at 01:00

Just to show some methods of generating a Fibonacci.py series that I know of:

class   Fib:

    def __init__(self, *f01):
        self.f0, self.f1 = f01

    # Calculate the n-th item of the Fibonacci series
    def fibRecursive(self, n):
        if n == 0:
            return self.f0
        elif n == 1:
            return self.f1
        return self.fibRecursive(n-2) + self.fibRecursive(n-1)

    def fibAppend(self, n):
        f = [self.f0, self.f1]
        for i in range(2, n):
            f.append(f[i-2]+f[i-1])
        return f

    def fibYield(self, n):
        x, y = self.f0, self.f1
        for _ in range(n):
            yield x
            x, y = y, x + y

    def fibIncremental(self, n):
        x, y = self.f0, self.f1
        f = [x, y]
        for i in range(2, n):
            x, y = y, x + y
            f.append(y)
        return f

    def fibLCAE(self, n):   # LC with Assignment expression
        f = [self.f0, self.f1]
        f += [(f := [f[1], f[0] + f[1]]) and f[1] for i in range(2,n)]
        return f

if  __name__ == '__main__':
    n = int(input("How many numbers in the Fibonacci sequence?   "))
    fd = Fib.__dict__
    for f in [Fib(0, 1), Fib(3, -3), Fib(-50, 100)]:
        print(f'Fibonacci initial two numbers : {f.f0}, {f.f1}')
        for func in [fd['fibAppend'], fd['fibIncremental'], fd['fibLCAE']]:
            print(func(f, n))
        print([f.fibRecursive(i) for i in range(n)])
        print(list(f.fibYield(n)))
        print()

>>> python.exe Fibonacci.py
How many numbers in the Fibonacci sequence?   13
Fibonacci initial two numbers : 0, 1
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144]
....
Fibonacci initial two numbers : 3, -3
[3, -3, 0, -3, -3, -6, -9, -15, -24, -39, -63, -102, -165]
....
Fibonacci initial two numbers : -50, 100
[-50, 100, 50, 150, 200, 350, 550, 900, 1450, 2350, 3800, 6150, 9950]
....

ChuckCottrill · Answer 3 · 2020-11-19T16:32:46.070

How about two list comprehensions, or build a base list, then compute?

Generate fibonacci base array,

fb = [n for n in range(0,21)]
# or
fb = [0] + [1] * 20

Calculate fib(13), (want: [f[n] := f[n-1] + f[n-2] for n in range(2,20)])

f = [ fib(fb,n) for n in range(2,20) ]

Where fib(f,n) is:

def fib(f,n):
    if n > 1:
        f[n] = f[n-1] + f[n-2]
    return f[n]

Fibonacci sequence:

fb[0:13]: [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144]
fib(fb,13-1) = 144

Can assignment expression create Fibonacci series using list comprehension?

3 Answers3