StreamPlot error "ndim" is not an attribute

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As asked, your question does not use any Sage specific function and uses Sage as a Python interpreter. It would probably better aimed at a Python list/forum.

But Sage has functions useful for this task. Given :

sage: var("Q,E0")
(Q, E0)
sage: K=1/(4*pi*E0) # Keep constant's definition.
sage: NumVals={Q:1,E0:8.854187817e-12} # Separate symbolic and numeric computations...
sage: f(x,y)=K*Q/sqrt(x^2+y^2) # define f symbolically.

You may compute :

sage: f.gradient()
(x, y) |--> (-1/4*Q*x/(pi*(x^2 + y^2)^(3/2)*E0), -1/4*Q*y/(pi*(x^2 + y^2)^(3/2)*E0))

i. e. a symbolic function of two variables returning a bidimensional vector, better typeset as :

$$ \left( x, y \right) \ {\mapsto} \ \left(-\frac{Q x}{4 \, \pi {\left(x^{2} + y^{2}\right)}^{\frac{3}{2}} E_{0}},\,-\frac{Q y}{4 \, \pi {\left(x^{2} + y^{2}\right)}^{\frac{3}{2}} E_{0}}\right) $$

Graphing needs numerical values, but having them in a dictionary allows to keep the working tools (definitions, equations, intermediate results...) symbolic (i. e. understandable) until numerical values are needed.

I'm not sure what Matplotlib's streamplot is supposed to do. I suppose its either :

sage: streamline_plot(f.gradient().subs(NumVals).list(), (-5, 5), (-5, 5))
Launched png viewer for Graphics object consisting of 1 graphics primitive

or, possibly,

sage: plot_vector_field(f.gradient().subs(NumVals), (x, -5, 5), (y, -5, 5))
Launched png viewer for Graphics object consisting of 1 graphics primitive

HTH,

I think the problem is that you're trying to mix numpy and matplotlib and Sage in ways that are not supported. The documentation for plt.streamplot says:

Parameters
**********

x, y : 1D/2D arrays
   Evenly spaced strictly increasing arrays to make a grid.  If 2D,
   all rows of *x* must be equal and all columns of *y* must be equal;
   i.e., they must be as if generated by "np.meshgrid(x_1d, y_1d)".

u, v : 2D arrays
   *x* and *y*-velocities. The number of rows and columns must match
   the length of *y* and *x*, respectively.

It looks like you're trying to pass grid in the place of both x and y, but grid is a function, not a pair of arrays of values. You also define x and y using np.meshgrid, but the later assignment eqn(x,y) = K*Q / (x**2 + y**2)^(1/2) redefines x and y to be symbolic variables. I suggest instead:

xx,yy = np.meshgrid (gridx, gridy) # don't redefine x and y

and

G = [grad(x,y) for (x,y) in zip(gridx,gridy)]
Gx = [a[0] for a in G]
Gy = [a[1] for a in G]

and finally

plt.streamplot (Gx, Gy, xx,yy, color='green')

This doesn't work, either, but the error message is different. Maybe you need to do more conversion to get xx and Gx in the same format (convert both to floats? convert Gx to a numpy array?).