list - Python: I don't understand the order of a function calling a function -

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i'm following book data science scratch joel grus , decribe following code create identity matrix

def make_matrix(num_rows, num_cols, entry_fn):     return [[entry_fn(i, j)              j in range(num_cols)]                        in range(num_rows)]  def is_diagonal(i, j):     return 1 if == j else 0  identity_matrix = make_matrix(5, 5, is_diagonal)

although can sort of see how create identity matrix, i'm having difficulties understanding it.

the way see call function make_matrix arguments 5, 5 , is_diagonal. @ point code go is_diagonal(i, j) j in range(5) , go function is_diagonal without having seen outer loop ... in range(5). if true, seems function is_diagonal input variable (0,j), (1,j), ..., (4,j) , is_diagonal doesn't enough input variables (because j isn't defined). please explain i'm going wrong in train of thoughts?

that type of expression (a comprehension) best of thought of in yoda sense: backwards is. last part of expression evaluated before first.

this function equivalent of:

def make_matrix(num_rows, num_cols, entry_fn):     ret = [] # dealing outer list      # , outer loop     in range(num_rows):         cur = [] # , inner list         # , inner loop         j in range(num_cols):             curr.append(entry_fn(i,j)) # add result inner list         # , once you're done inner loop, , result outer list         ret.append(cur)      # finally, complete outer loop , return result     return ret

both i and j exist in greater context of function, though i defined after j in more compressed version.


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