Our example program checktext.py asks the user for a text file and checks what style end-of-lines the file has. This may need a little explanation: ASCII text files are almost identical on different machines, with one exception:
0x0a,
0x0d and
macfs.PromptGetFile. This is one of the routines
that allow you to ask the user to specify a file. You pass it one required
argument, the prompt string. There are up to four optional MacOS file type arguments
you can pass, as 4-byte strings. Specifying no file
type will allow the user to select any file, specifying one or more types restricts
the user to files of this type. File types are explained in most books on the Mac.
PromptGetFile returns two values: an FSSpec object and a
success indicator. The FSSpec object is the "official" MacOS way of specifying a
file, more on it later. The success indicator tells you whether the user clicked OK
or Cancel. In the event of Cancel we simply exit back to the finder.
PromptGetFile has a number of friends that do similar things:
StandardGetFile is identical to PromptGetFile but
without the prompt. It has up to four optional filetype arguments.
StandardPutFile asks the user for an output file. It will
warn the user when she tries to overwrite an existing file. The routine has one
mandatory argument: a prompt string. Pass the empty string if you do not want a prompt.
GetDirectory asks the user for a folder (or directory, in unix terms).
It has one optional argument: a prompt string.
There are many things you can do with FSSpec objects (see the
macfs section in the
Python Library Reference
for details), but passing them to open is not
one of them. For this, we first have to convert the FSSpec object to a pathname, with
the as_pathname method. This returns a standard MacOS-style pathname with
colon-separated components. This can then be passed to open. Note that
we call open with mode parameter 'rb': we want to read the file in binary
mode. Python, like C and C++, uses unix-style line endings internally and opening a
file in text mode ('r') would result in conversion of carriage-returns to
linefeeds upon reading. This is something that Mac and DOS programmers are usually aware
of but that never ceases to amaze unix buffs.
After we open the file we attempt to read all data into memory. If this fails we use
EasyDialogs.Message to display a message in a standard dialog box and exit.
The EasyDialogs module has a few more useful simple dialog routines, more on that in
example 1.
The rest of the code is pretty straightforward: we check that the file actually contains data, count the number of linefeeds and returns and display a message with our guess of the end-of-line convention used in the file.
The example0 folder has three text files in Mac, Unix and DOS style for you to try the program on. After that, you can continue with example 1 or go back to the index to find another interesting topic.