*(The OP only loosely specified the input & output method/format, so the following seems acceptable.)*

Sage Notebook (<s>39 41</s> 40)
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In the current cell,

    f=lambda n,k:k<1or f(n-1,k-1)*n/k;+f(*_)

where the input in the form `n,k` is entered & evaluated in the preceding cell. This simulates "command-line input" by assigning it to `_` (similar to command-line arguments).

Sage Notebook (<s>42 44</s> 43)
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Alternatively, using "in-source input" (with only the `x=` and newline characters adding to the score), e.g.,

    x=5,2
    f=lambda n,k:k<1or f(n-1,k-1)*n/k;+f(*x)

*Both of these approaches are obviously spin-offs from earlier answers by others.*