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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def read_first_available_value(filename, field_name):
 """Reads the first assigned value of the given field in the CSV table. """

    if not os.path.exists(filename):
        return None
    with open(filename, 'rb') as csvfile:
        reader = csv.DictReader(csvfile)
        for row in reader:
            value = row.get(field_name)
            if value:
                return value
    return None