Should I read  ingredient weight as prepared or unprepared weight? Once I have made a recipe a couple of times, I tend to follow my instincts rather than strict portion sizes.

When following a new recipe, however, I have often wondered about how an ingredient's weight should be interpreted.

For example, if a recipe calls for 500g of pumpkin, diced into 1cm cubes, does convention expect me to use a 500g cut of pumpkin, that I then de-seed and dice, or 500g of 1cm pumpkin cubes pumpkins?
In your example, it means 500g pumpkin before preparing. 

In general, you might see this two ways:


500g pumpkin. Dice the pumpkin.
500g diced pumpkin.


The first option refers to the weight before prepping while the second refers to the weight after prepping. In general, the second one is far more exact for the actual recipe while the first one is more exact for shopping. I assume ease of shopping plus presumption of yield is why the first one is so often used, although in high end cook books (Grant Achatz, Thomas Kellar, etc) the weights are often given post prep for precision. 

You will see the same thing for volume measurements (i.e. one cup of nuts chopped vs one cup of chopped nuts).