[Tutor] pass tuples to user defined function(beginner)

Alan Gauld alan.gauld at btinternet.com
Wed Nov 30 02:16:14 CET 2011

On 29/11/11 21:40, Mayo Adams wrote:

> tuple that is in the file, it would be helpful to know what it is.
> Presumably, a string representing a tuple.

Exactly so, everything in the file is a string. If it's a binary file it 
is a string of bytes, if it is a text file a string of characters. The 
interpretation of those bytes or characters is down to the reading 
program. The program must convert the string into data. The string 
represents the data that will be eventually created in the computer's 
memory within the interpreter's execution environment.

> representation,  I cant immediately see how anything in a script is
> anything other than a representation of some kind, hence the
> distinction between representamen and object does no work for me.

While it is in the file it is just a string. a string representation of 
a program. The interpreter reads that string and converts it into symbol 
tables, executable objects and data structures. Your program, when it 
reads a file must do exactly the same, it must convert the string read 
from the file into whatever data structure it represents and, if 
necessary, assign that data to a variable for future reference.

The process of reading a string and interpreting its meaning as data is 
what's known as parsing. There are several parsers available in Python 
to help with that task, as well as some tools to help you write your own 
parser. Examples are the csv module, HTMLParser, xpat and ElementTree. 
But even with these you will often have to do the final type conversion 
from string to integer or Boolean etc yourself.

The other option is to use binary files where the data is stored in 
strings of bytes. In this case you still need to interpret the string
but you do so by reading the exact number of bytes needed to store the 
particular data type, for example 4 bytes for a 32 bit integer. You then 
have to convert those 4 bytes into an integer. Again Python offers some 
help in the form of the struct module. Binary files are usually more 
compact than text files storing the same information, but they are far 
harder to interpret. There are other standard types of binary file and 
Python offers support for some, most notably there are libraries to read 
Excel files, various databases and PDF documents. Most of these 
libraries will also contain functions that help you write data back out 
to the files in the same format as well.

The goal of all of these libraries is to make it comparatively easy to 
read a string representation from a file and convert it into real data 
that we can manipulate in our programs.


Alan G
Author of the Learn to Program web site

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