pygame error, No video mode has been set

I am just setting up some functions for my game but my script fails to load the image

#used variables
# x, y for alien location
# nPc for the aliens image
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#set up
import pygame, sys, random, time, math
from pygame.locals import *
pygame.init()


nPc = '/home/claude/Dropbox/Bowtie/Prisim/Images/Alien_Races/Standered/alien_1.png'


nPc = pygame.image.load(nPc).convert_alpha()

def loc_alien():
        x = random.randint(0, 400)
        y = randaom.randint(0, 400)


def spawn_alien(x, y):
        screen.blit(nPc, (x, y))

when I run this I wont nothing to happen as I am not using the functions yet, but when I do run it I get this error

Traceback (most recent call last):
  File "/home/claude/Dropbox/Bowtie/Prisim/Scripts/aliens.py", line 26, in <module>
    nPc = pygame.image.load(nPc).convert_alpha()
error: No video mode has been set

anyone know what I'm doing wrong?

I believe that you need to call:

screen = pygame.display.set_mode((800, 600)) # change to the real resolution

this call will actually return the surface that you want to blit on. Below is the documentation from the linked resource.

pygame.display.set_mode()

Initialize a window or screen for display
set_mode(resolution=(0,0), flags=0, depth=0) -> Surface
This function will create a display Surface. The arguments passed in are requests for a display type. The actual created display will be the best possible match supported by the system.

The resolution argument is a pair of numbers representing the width and height. The flags argument is a collection of additional options. The depth argument represents the number of bits to use for color.

The Surface that gets returned can be drawn to like a regular Surface but changes will eventually be seen on the monitor.

If no resolution is passed or is set to (0, 0) and pygame uses SDL version 1.2.10 or above, the created Surface will have the same size as the current screen resolution. If only the width or height are set to 0, the Surface will have the same width or height as the screen resolution. Using a SDL version prior to 1.2.10 will raise an exception.

It is usually best to not pass the depth argument. It will default to the best and fastest color depth for the system. If your game requires a specific color format you can control the depth with this argument. Pygame will emulate an unavailable color depth which can be slow.

When requesting fullscreen display modes, sometimes an exact match for the requested resolution cannot be made. In these situations pygame will select the closest compatible match. The returned surface will still always match the requested resolution.

The flags argument controls which type of display you want. There are several to choose from, and you can even combine multiple types using the bitwise or operator, (the pipe “|” character). If you pass 0 or no flags argument it will default to a software driven window. Here are the display flags you will want to choose from:
pygame.FULLSCREEN    create a fullscreen display
pygame.DOUBLEBUF     recommended for HWSURFACE or OPENGL
pygame.HWSURFACE     hardware accelerated, only in FULLSCREEN
pygame.OPENGL        create an OpenGL renderable display
pygame.RESIZABLE     display window should be sizeable
pygame.NOFRAME       display window will have no border or controls
For example:
# Open a window on the screen
screen_width=700
screen_height=400
screen=pygame.display.set_mode([screen_width,screen_height])