How do I import a lower triangular matrix using pandas?
The straightforward answer is that you don't simply import the triangle: its shape is, by definition, incompatible with the rectangular format required by the built-in readers of NumPy and PANDAS.  You have to write your own code to read the input file, stuff the values into the corresponding rows of your chosen data structure, and fill the missing columns with your default value: zeroes, I assume.  This needs only a simple loop.
Given that what you want is not a built-in method, can you handle the coding from here?
I am trying to import a lower triangular matrix into pandas (or numpy) in Python in order to convert it into a symmetrical square matrix but I am stuck at the import step.
The matrix has the first column with names but the rest of the columns are numbers in a tab delimited format like this:

 A
 B   1 
 C  10  20
 D  21  25  45

I get error when I try to import it using numpy:

myfile = np.genfromtxt("trimatrix.txt", delimiter="\t")

and also when I use pandas:

myfile = pd.read_table("trimatrix.txt")

In both cases, the errors are because there is only one column after reading the first row but the other rows have more columns.
Thank you for your time and help!