[cairo] measure needed canvas size

[Sam Tygier]

hi

i want to draw a diagram, 2d CAD layout sort of thing. i have a list of various 
shapes and lines and their positions and rotations in physical space. and i have 
a function that takes this list and draws it to a context of a surface (pdf or 
png).

i'd like a way to know the maximum required space, so that i can choose a size 
for the canvas. currently i have a function that goes through the list and works 
out the coordinate of each point that needs to be drawn, and keeps a max_x, 
max_y, min_x, min_y. this function is very like my drawing function, which seems 
wasteful. also when the 2 functions diverge (when i add a new shape to the draw 
function) then the bounds get messed up.

i am now wondering about making a dummy context object that ignored all but a 
few functions (move_to(), line_to()), and instead of drawing anything, just 
records  max_x, max_y, min_x, min_y.

this seems to me like it must be a common problem. is there a better solution? 
is there already such a dummy context?

i am using cairo  though python, if that makes a difference.

thanks

sam