How to make a function of 'fname' that produce a file named 'fname'

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Hi all,

Typing a correction on some recurrent sequences $u_{n+1}=f(u_n)$ I use sage to build a nice plot illustrating the behavior of the sequence depending on the initial data. I took a nice function from this awesome book.

def escargot(f,x,u0,n,xmin,xmax,file):
    u = u0
    P = plot(x, x, xmin, xmax, color='gray')
    for i in range(n):     
        P+= line([[u,u],[u,f(u)],[f(u),f(u)]], color ='red')
        u = f(u)
    P += f.plot(x, xmin, xmax, color='blue') 
    P.save('/home/myfolder/f1a.png')

So everyhting work perfectly but, since there is a lot of examples to produce, I would like to make the function to have an extra argument say filename so that now the file so that now the plot is saved in /home/myfolder/filename.png, but I have no idea how sage manipulate string. Help would be welcome !