What does a underscore before a letter (_p) mean in python

Question

I run into a function but not quite understand it. I am not sure it is a convention or has a meaning. what does the _p , where did the _p enter the function. It would be much appreciated if you can give me some explanation on the for loop here.

    def contraction_mapping(S, p, MF, params, beta=0.75, threshold=1e-6, suppr_output=False):
        ''' 
Initialization of the state-transition matrices: 
        describe the state-transition probabilities if the maintenance cost is incurred, 
        and regenerate the state to 0 if the replacement cost is incurred.
        '''
        ST_mat = np.zeros((S, S))
        p = np.array(p) 
        for i in range(S):
            for j, _p in enumerate(p):  
                if i + j < S-1:
                    ST_mat[i+j][i] = _p

                elif i + j == S-1:
                    ST_mat[S-1][i] = p[j:].sum()
                else:
                    pass

        R_mat = np.vstack((np.ones((1, S)),np.zeros((S-1, S))))        

Answer 1

See PEP8 for details on many python style conventions. In particular, you can find the description of a single leading underscore here:

https://www.python.org/dev/peps/pep-0008/#descriptive-naming-styles

_single_leading_underscore : weak "internal use" indicator. Eg from M import * does not import objects whose name starts with an underscore.

In the loop above, this is somewhat of a misuse, as they are only using _p to avoid clashing with the existing name p . These variable names are not great obviously. The _p are items of the array provided by enumerate, while p is the entire array as well (locally overriding the original p parameter passed in).

As a side note, the loop itself is somewhat awkward, and could be simplified and optimized (mostly fallout from using better ranges instead of pass , and avoiding recalculating the sum repeatedly).