Issues with passing variables through function in python

Question

I with difficult to understand how the function rhs(u, m, r) from the code below is receiving the parameters m and r. As it is possible to see from the code, the function rhs is called inside the function euler_step(u, rhs, dt), however neither the parameters m and r is passed as arguments to the function euler_step nor they are global variable. So, someone can explain to me how the parameters m and u are arriving go the function rhs.

# model parameters:
mpo = 100.   # initial mass of the rocket propellant in kg
ms = 50.     # mass of the rocket shell in kg
g = 9.81     # gravity in m s^{-2}
rho = 1.091  # average air density in kg/m^{3}
rad = 0.5    # radius of the maximum cross sectional area of the rocket in m
A = numpy.pi*(rad**2)# maximum cross sectional area of the rocket in m^{2}
v_e = 325.   # the exhaust speed in m/s
C_D = 0.15   # drag coefficient
rt = 20.0    # propellant burn rate in kg/s
dtp = 5.0    # time interval to empty the propellant in s

### set initial conditions ###
h0 = 0.0 # start at the zero height [m]
v0 = 0.0 # initial speed [m/s]

def rhs(u, m, r):
    """Returns the right-hand side of the phugoid system of equations.

    Parameters
    ----------
    u : array of float
        array containing the solution at time n.
    mp: float
        mass of the propellant at time t
    mp_rate: float
        propellant burn rate

    Returns
    -------
    dudt : array of float
        array containing the RHS given u.
    """
    print("[m,r]",[m,r])
    [h,v] = u.copy()

    return numpy.array( [ v, -g + pow((ms+m),-1)*(r*v_e - 0.5*rho*v*abs(v)*A*C_D) ] )

def euler_step(u, rhs, dt):
    """Returns the solution at the next time-step using Euler's method.

    Parameters
    ----------
    u : array of float
        solution at the previous time-step.
    rhs : function
        function to compute the right hand-side of the system of equation.
    dt : float
        time-increment.

    Returns
    -------
    u_n_plus_1 : array of float
        approximate solution at the next time step.
    """

    return u + dt * rhs(u, m, r)

if __name__ == "__main__":
    T = 17.0                     # final time
    dt = 0.1                     # time increment
    t = numpy.arange(0.0, T, dt) # time discretization
    N = len(t)                   # number of time-steps

    # initialize the array containing the solution for each time-step
    u = numpy.zeros((N, 2))
    u[0] = numpy.array([h0, v0]) # fill 1st element with initial values
    rate = numpy.zeros(N)
    mp = numpy.zeros(N)
    Np = int(((N)/(T))*dtp) # number of time-steps with propellant burn

    rate[0:Np] = rt # propellant burn rate in kg/s
    mp[0:Np] = mpo - rt*t[0:Np]

    # time loop - Euler method
    for n in range(1,N-1):

        r = rate[n]
        m = mp[n]
        print("[R,M]",[r,m])
        u[n+1] = euler_step(u[n], rhs, dt)

Thanks in advance.

euler_step is passing the global variable m and r to rhs function. — Tadhg McDonald-Jensen
– Tadhg McDonald-Jensen, Commented Mar 9, 2016 at 16:32

Juan Leni · Accepted Answer · 2016-03-09 16:32:48Z

4

m and n are globals.

It can be confusing because it may seem that __main__ was a function but it is not. The if __name__ == "__main__" .... is running on a global scope.

answered Mar 9, 2016 at 16:32

Juan Leni

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1 Comment

Randerson Over a year ago

Other question: Why it is used [h,v] = u.copy(), to the get value of u and not simply a normal assignment [h,v] = u???

Community · Accepted Answer · 2017-05-23 11:45:16Z

3

They are global variables. In Python, if, while and for do not create a separate variable scope, so they're still assigned values in global/module scope prior to first invocation of euler_step:

if __name__ == "__main__":  # does not start a new variable scope
    ...

    for n in range(1,N-1):  # does not start one either
        # thus these variables are set in global scope.
        r = rate[n]
        m = mp[n]

        # and euler_step is invoked only here, thus it will see
        # r and m being set.
        u[n+1] = euler_step(u[n], rhs, dt)

See also Short Description of the Scoping Rules?.

edited May 23, 2017 at 11:45

CommunityBot

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answered Mar 9, 2016 at 16:32

Antti Haapala

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Matt S · Accepted Answer · 2016-03-09 16:34:08Z

1

m and r are defined near the bottom of your script at the module level:

r = rate[n]
m = mp[n]

Therefore they are available to all functions within the module:

The following are blocks: a module, a function body, and a class definition. A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one, unless a contained block introduces a different binding for the name... When a name is used in a code block, it is resolved using the nearest enclosing scope.

answered Mar 9, 2016 at 16:34

Matt S

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