[Tutor] Scope Problem with Files

[bob]

At 11:13 AM 12/14/2005, Carroll, Barry wrote:
>Greetings:
>
>I am implementing a (crude but useful) debug facility in my test 
>system client software.  Basically, I test the value of a global 
>Boolean. It True, I write pertinent data to a text file.  I want to 
>do this in multiple functions in a module.  Rather than open and 
>close the file each time I write, I want to open the file once at 
>the start of process and close it at the end.  Here are excerpts 
>from the module.
>
>##########################
>import socket
>import struct
>import time
>
># initialize the debug flag
>DEBUG = True
>. . .
>
>dbgf = None                 # File object and path for saving debug output
>dbgfname = "debug.txt"
>
>def snd_cmd(sock,cmd):
>
>     . . .
>
>     while remainlen > 0:
>         if remainlen > MTU:
>             pktstp = pktstrt + MTU
>         else:
>             pktstp = pktlen
>             pktflags |= EOD
>
>         pkthdr = struct.pack('@2BH',pktflags,seq,pktlen)
>         sndpkt = pkthdr+cmd[pktstrt:pktstp]
>
>         if DEBUG:
>             dbgf.write("command: " + cmd + "\n")
>             dbgf.write("flags: 0x%X, seq: %u, len: %u\n" % 
> (pktflags, seq, pktlen))
>
>         sock.sendto(sndpkt,addr)
>
>         pktstrt += MTU
>         remainlen -= MTU
>         pktflags = pktflags & ~SOD
>         seq = (seq + 1) % 256
>
>     . . .
>
>def recv_resp(sock):
>     response = ''
>     try:
>         response = sock.recv(MTU)
>     except socket.timeout:
>         errtupl = ("ERROR", 'Server did not respond')
>         return (errtupl, response)
>
>     . . .
>
>     if DEBUG:
>         dbgf.write("response: " + response + "\n")
>         dbgf.write("flags: 0x%X, seq: %u, len: %u\n" % (flags, retseq, dlen))
>
>     . . .
>
>     return (errtupl, response)
>
>def do_cmd(cmd):
>
>     sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
>     sock.settimeout(timetowait)
>     retriesleft = retries
>     if DEBUG:
>         dbgf = open(dbgfname,mode="a")

dbgf is a local variable. If you want to reassign to the global you must add
global dbgf
to the function

>         dbgf.write("\n"+str(time.localtime())+"\n")
>
>     while retriesleft > 0:
>         snd_cmd(sock, cmd)
>         recvtupl = recv_resp(sock)
>         if recvtupl[0][0] != "ERROR":
>             break
>         retriesleft -= 1
>
>     if DEBUG:
>         dbgf.close()
>
>     sock.close( )
>     return recvtupl
>##########################
>
>When I run this code, I get the following error message:
>
><<<<<<<<<<<<<<<<<<<<<<<
>A problem occurred in a Python script. Here is the sequence of 
>function calls leading up to the error, in the order they occurred.
>/var/www/cgi-bin/pagen.py
>    76 # function.  Check the returned error code for success.
>    77 cmdtosnd = state['s']['getcmd']
>    *****78 (errorcode, errorstr), 
> platformstate['itype']['curstate'] = do_cmd(cmdtosnd)
>    79 if errorcode == 0:
>    80     cmdtosnd = state['t']['getcmd']
>
>     . . .
>
>/var/www/cgi-bin/Client.py in do_cmd(cmd='cmd')
>   160
>   161     while retriesleft > 0:
>   *****162         snd_cmd(sock, cmd)
>   163         recvtupl = recv_resp(sock)
>   164         if recvtupl[0][0] != IIPSRVERROR:
>
>global snd_cmd = <function snd_cmd>, sock = <socket._socketobject 
>object>, cmd = 'cmd'
>
>
>  /var/www/cgi-bin/Client.py in snd_cmd(sock=<socket._socketobject 
> object>, cmd='cmd')
>    65
>    66         if DEBUG:
>    *****67             dbgf.write("command: " + cmd + "\n")
>    69
>
>global dbgf = None, dbgf.write undefined, cmd = 'cmd'
>
>
>AttributeError: 'NoneType' object has no attribute 'write'
>       args = ("'NoneType' object has no attribute 'write'",)
> >>>>>>>>>>>>>>>>>>>>>>>
>
>dbgf is declared at the top of the module.  It is opened and closed 
>in do_cmd.  I attempt to write to it in snd_cmd and recv_resp, both 
>of which are called by do_cmd.  Since dbgf is global to all of these 
>functions, I expected its value (the open file object) to 
>persist.  I don't understand why it didn't.  I expect I have 
>misunderstood Python's scoping rules.  Can someone enlighten me?
>
>Thanks and enjoy the holidays.
>
>BGC
>________________________
>"Never trust anything that can think for itself
>if you can't see where it keeps its brain"
>JK Rowling
>
>
>
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