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Both repr and str aim to provide human-readable string representations.

They coincide for small Sage matrices, but differ for matrix size above 20.

Indeed, try the following:

sage: a = zero_matrix(ZZ, 20)
sage: a
20 x 20 dense matrix over Integer Ring (use the '.str()' method to see the entries)
sage: repr(a)
'20 x 20 dense matrix over Integer Ring'
sage: str(a)
'[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]\n[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]'

Some Sage objects have a sage_input method giving code to recreate them.

It is the case for integer matrices:

sage: M = matrix([[1, 1], [1, 1]])
sage: sage_input(M)
matrix(ZZ, [[1, 1], [1, 1]])