some cares about compiler warnings

[cxzhong]

• fix some variables which are not init
• fix some compare with different types of integers.
• fix some mismath like def to cpdef to match the header file

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[dcoudert]

I can not check all default values you assigned, in particular in code/methods that I don't know.

[dcoudert]

there is something wrong in this method. if xmax is None, then x_max is set to self.n. Otherwise, it will remain to the default value you assigned, that is 0.
Why not doing like:

if xmin is None:
    xmin = 0
if xmax is None:
    xmax = self.n
for i in range(xmin, xmax):
...

[dcoudert]

this cannot be initialized to 0. IT should be +oo

[dcoudert]

should be initialized to large value +oo