[Tutor] reading configuration file

[Gonçalo Rodrigues]

On Tue, 11 Nov 2003 11:07:45 +0100, you wrote:

>Hi all
>
>I've made a prog that reads a configuration file. While it works, I think it is
>butt-ugly and defintely not the way it should be done in Python.
>
>I tried to understand dictionaries and I tried to make eval() work. I failed. 
>What is the good Python way to do what I do below?
>
>config file:
>
>>-----------------------------------
># parameter settings for pump board and fan boards
># from six fans and pump controller
>
># pump controller settings
>CO2_soll		 80
>pump_1_manual_override    0  
>pump_1_manual_speed     120
>pump_2_manual_override    0  
>pump_2_manual_speed     130 
>
># Fan board settings:
>fan_manual_override    0  
>fan_manual_speed     140 
>>-----------------------------------
>

As you mention the best way to do this is to fill a dictionary where
the keys are the "titles" and the values the numbers.

Let me first fire up the interpreter and show you a typical use of a
dictionary:

>>> #Create an empty dictionary.
>>> dct = {}
>>> print dct
{}
>>> #Add a key, value pair.
>>> dct["my key"] = "my value"
>>> print dct
{'my key': 'my value'}
>>> #Rebind the associated value of the key.
>>> dct["my key"] = 0
>>> print dct
{'my key': 0}
>>> 

This is enough for what we want to do. The idea is

- read file
- parse the contents into a dictionary

From what I gleaned from your code there are also default values,
filled up at initialization time. We can start there and instead of
having a bunch of global variables we just stuff everything in a
(global) dictionary. As in:


DEFAULTDICT = {"CO2_soll": 81,
               "pump_1_manual_override": 1,
               "pump_1_manual_speed": 121,
               "pump_2_manual_override": 1,
               "pump_2_manual_speed": 131,
               "fan_manual_override": 1,
               "fan_manual_speed": 141}

We use capital letters to signal that this is a constant and the user
should not modify this dictionary in any way.

Now we want to code a function, call it parse, that receives the
*contents* of a file, a string, and parses it. A rule of thumb is that
each function does only one thing. As such we *do not* put things like
opening files and handling IO error conditions inside it.

The usage of the function we envision is as in

#Open file: filename is... the filename.
f = file(filename)
try:
    #Read whole file as one string.
    data = f.read()
finally:
    #Close file
    f.close()
#Parse data string.
config = parse(data)


Now we can go on with the function. Since we receive data as a string
we must first split it in lines and then we go through each line
reading it, putting the values inside the dictionary.

def parse(s):
    """Parse a string into a dictionary."""
    #Fetch a *copy* of the default dictionary.
    ret = DEFAULTDICT.copy()
    #Split lines.
    lines = s.split("\n")
    #Loop through lines.
    for line in lines:
        #Strip whitespace.
        line = line.strip()
        #Skip comment and blank lines.
        if line and line[0] != "#":
            #Split the line in the pair key, value
            values = line.split()
            #Fill dictionary.
            ret[values[0]] = int(values[1])
    #Return dictionary.
    return ret

The above function is commented all the way so you should be able to
see what is going on.

With my best regards,
G. Rodrigues