Python newbie question re Strings and integers

Bruno Desthuilliers bdesth.quelquechose at free.quelquepart.fr
Sat Sep 20 18:54:37 CEST 2008


rmac a écrit :
> 
> the following code attempts to extract a symbol name from a string:
>     extensionStart = int(filename.rfind('.'))

rfind returns an int, so passing it to the int type constructor is useless.

>     filenameStart = int(filename.rfind('/'))

idem

>     #print 'Extension Start - ' + str(extensionStart)

The print statement isn't restricted to string objects. And FWIW, you 
can use string formating. The above should be either

   print "Extension start - ", extensionStart

or

   print "Extension start - %s" % extensionStart

As a side not, the naming convention in Python is to use all_lower for 
identifiers.

>     #print 'FileName Start - ' + str(filenameStart)

idem.

>     currentSymbol=filename[int(filenameStart),int(extensionStart)]

Two more useless int constructor calls.

> Uncommenting the print statements clearly show the values to be
> integers

If by 'the values', you mean objects bound to identifiers 
'filenameStart' and 'extensionStart', I fail to see how they could be 
anything else...

> (and without the str casts actually provide int+string
> errors)

Indeed. What should be the semantic of expression
   'answer is ' + 42

???

> However, executing this code results in...
>     opening - /Users/rmac/Documents/Sandbox/data/MarketData/AA.csv
>     Traceback (most recent call last):
>       File "rHistFileToDB_Equities.py", line 25, in <module>
>         currentSymbol=filename[int(filenameStart),int(extensionStart)]
>     TypeError: string indices must be integers

the expression:

    int(filenameStart),int(extensionStart)

1/ takes the object currently bound to identifier 'filenameStart' and 
pass it to the int type constructor

2/ takes the object currently bound to identifier 'extensionStart' and 
pass it to the int type constructor

3/ build a tuple (actually, a pair of ints) from the results of the two 
above expressions



Then you try to use this tuple as an index on a string. A tuple is not a 
legal type for string (or any other sequence) subscripting.

The complete syntax for string subscripting is described in the 
FineManual. To make a long story short, it's :

     string[start:end:step]

where end and step are optionals.

IOW, the correct syntax here is:
     filename[filenameStart:extensionStart]


Now, while this is the correct *syntax*, it's not the correct *idiom*. 
There's a module named os.path, which provides the needed service. I 
leave it up to you to read the relevant part of the documentation...



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