This is the 11th script in the series Python Scripts and second script where we are creating something using turtle.

Code is self explanatory as I have added plenty of comments within code. 

Before proceeding further, it is important to understand the coordinates and quadrants. Refer the diagram below.

We have taken the flag dimensions ratio into consideration while writing program. Still if there are any inaccuracies, please consider this tutorial for educational purpose only.

Code:

#
# Python script to create USA flag using turtle.
# Author - https://www.pythoncircle.com
# Original Author - https://www.codesters.com/preview/6fe8df0ccc45b1460ab24e5553497c5684987fb5/

import turtle
import time

# create a screen
screen = turtle.getscreen()
# set background color of screen
screen.bgcolor("white")
# set tile of screen
screen.title("USA Flag - https://www.pythoncircle.com")
# "Yesterday is history, tomorrow is a mystery, 
# but today is a gift. That is why it is called the present.”
# — Oogway to Po, under the peach tree, Kung Fu Panda Movie
oogway = turtle.Turtle()
# set the cursor/turtle speed. Higher value, faster is the turtle
oogway.speed(100)
oogway.penup()
# decide the shape of cursor/turtle
oogway.shape("turtle")

# flag height to width ratio is 1:1.9
flag_height = 250
flag_width = 475

# starting points
# start from the first quardant, half of flag width and half of flag height
start_x = -237
start_y = 125

# For red and white stripes (total 13 stripes in flag), each strip width will be flag_height/13 = 19.2 approx
stripe_height = flag_height/13
stripe_width = flag_width

# length of one arm of star
star_size = 10


def draw_fill_rectangle(x, y, height, width, color):
    oogway.goto(x,y)
    oogway.pendown()
    oogway.color(color)
    oogway.begin_fill()
    oogway.forward(width)
    oogway.right(90)
    oogway.forward(height)
    oogway.right(90)
    oogway.forward(width)
    oogway.right(90)
    oogway.forward(height)
    oogway.right(90)
    oogway.end_fill()
    oogway.penup()

def draw_star(x,y,color,length) :
    oogway.goto(x,y)
    oogway.setheading(0)
    oogway.pendown()
    oogway.begin_fill()
    oogway.color(color)
    for turn in range(0,5) :
        oogway.forward(length)
        oogway.right(144)
        oogway.forward(length)
        oogway.right(144)
    oogway.end_fill()
    oogway.penup()


# this function is used to create 13 red and white stripes of flag
def draw_stripes():
    x = start_x
    y = start_y
    # we need to draw total 13 stripes, 7 red and 6 white
    # so we first create, 6 red and 6 white stripes alternatively    
    for stripe in range(0,6):
        for color in ["red", "white"]:
            draw_fill_rectangle(x, y, stripe_height, stripe_width, color)
            # decrease value of y by stripe_height
            y = y - stripe_height            

    # create last red stripe
    draw_fill_rectangle(x, y, stripe_height, stripe_width, 'red')
    y = y - stripe_height


# this function create navy color square
# height = 7/13 of flag_height
# width = 0.76 * flag_height
# check references section for these values
def draw_square():
    square_height = (7/13) * flag_height
    square_width = (0.76) * flag_height
    draw_fill_rectangle(start_x, start_y, square_height, square_width, 'navy')


def draw_six_stars_rows():
    gap_between_stars = 30
    gap_between_lines = stripe_height + 6
    y = 112
    # create 5 rows of stars
    for row in range(0,5) :
        x = -222
        # create 6 stars in each row
        for star in range (0,6) :
            draw_star(x, y, 'white', star_size)
            x = x + gap_between_stars
        y = y - gap_between_lines


def draw_five_stars_rows():
    gap_between_stars = 30
    gap_between_lines = stripe_height + 6
    y = 100
    # create 4 rows of stars
    for row in range(0,4) :
        x = -206
        # create 5 stars in each row
        for star in range (0,5) :
            draw_star(x, y, 'white', star_size)
            x = x + gap_between_stars
        y = y - gap_between_lines

# start after 5 seconds.
time.sleep(5)
# draw 13 stripes
draw_stripes()
# draw squares to hold stars
draw_square()
# draw 30 stars, 6 * 5
draw_six_stars_rows()
# draw 20 stars, 5 * 4. total 50 stars representing 50 states of USA
draw_five_stars_rows()
# hide the cursor/turtle
oogway.hideturtle()
# keep holding the screen until closed manually
screen.mainloop()

Copy the above code in usa_flag.py file and run it using command python3 usa_flag.py. 

If you get below error while running the script, you need to install python-tk.

raise ImportError, str(msg) + ', please install the python-tk package'
ImportError: No module named _tkinter, please install the python-tk package

sudo apt-get install python-tk

Code is available on github.

Video:

References: 

[1] https://coolpythoncodes.com/python-turtle/ 
[2] https://docs.python.org/3.6/library/turtle.html