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Background: I am a complete beginner when it comes to servers, but I know my way around programming in Python.
I am trying to setup a simple server using the basic Python 2.7 modules (SimpleHTTPServer, CGIHTTPServer, etc). This server needs to load a global, read-only variable with several GB of data from a file when it starts; then, when each user accesses the page, the server uses the big data to generate some output which is then given to the user.
For the sake of example, let's suppose I have a 4 GB file names.txt which contains all possible proper nouns of English:
Jack
John
Allison
Richard
...
Let's suppose that my goal is to read the whole list of names into memory, and then choose 1 name at random from this big list of proper nouns. I am currently able to use Python's native CGIHTTPServer module to accomplish this. To start, I just run the CGIHTTPServer module directly, by executing from a terminal:
python -m CGIHTTPServer
Then, someone accesses www.example-server.net:8000/foo.py and they are given one of these names at random. I have the following code in foo.py:
#!/usr/bin/env python
import random
name_list = list()
FILE = open('names.txt','r')
for line in FILE:
name = line[:-1]
name_list.append(name)
FILE.close()
name_to_return = random.choice(name_list)
print "Content-type: text/html"
print
print "<title>Here is your name</title>"
print "<p>" + name_to_return + "</p>"
This does what I want; however, it is extremely inefficient, because every access forces the server to re-read a 4 GB file.
How can I make this into an efficient process, where the variable name_list is created as global immediately when the server starts, and each access only reads from that variable?
881
Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function. To create a global variable inside a function, you can use the global keyword.
Global Variables In Python, a variable declared outside of the function or in global scope is known as a global variable. This means that a global variable can be accessed inside or outside of the function.
In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function's body, it's assumed to be a local unless explicitly declared as global.
It is also possible to use a global and local variable with the same name simultaneously. Built-in function globals() returns a dictionary object of all global variables and their respective values. Using the name of the variable as a key, its value can be accessed and modified.
Just for future reference, if anyone ever faces the same problem: I ended up sub-classing CGIHTTPServer's request handler and implementing a new do_POST() function. If you had a working CGI script without global variables, something like this should get you started:
import CGIHTTPServer
import random
import sys
import cgi
class MyRequestHandler(CGIHTTPServer.CGIHTTPRequestHandler):
global super_important_list
super_important_list = range(10)
random.shuffle(super_important_list)
def do_POST(s):
"""Respond to a POST request."""
form = cgi.FieldStorage(fp=s.rfile,headers=s.headers,environ={'REQUEST_METHOD':'POST','CONTENT_TYPE':s.headers['Content-Type'],})
s.wfile.write("<html><head><title>Title goes here.</title></head>")
s.wfile.write("<body><p>This is a test.</p>")
s.wfile.write("<p>You accessed path: %s</p>" % s.path)
s.wfile.write("<p>Also, super_important_list is:</p>")
s.wfile.write(str(super_important_list))
s.wfile.write("<p>Furthermore, you POSTed the following info: ")
for item in form.keys():
s.wfile.write("<p>Item: " + item)
s.wfile.write("<p>Value: " + form[item].value)
s.wfile.write("</body></html>")
if __name__ == '__main__':
server_address = ('', 8000)
httpd = CGIHTTPServer.BaseHTTPServer.HTTPServer(server_address, MyRequestHandler)
try:
httpd.serve_forever()
except KeyboardInterrupt:
sys.exit()
Whenever someone fills out your form and performs a POST, the variable form will be a dictionary-like object with key-value pairs which may differ for each user of your site, but the global variable super_important_list will be the same for every user.
Thanks to everyone who answered my question, especially Mike Steder, who pointed me in the right direction!
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