Using Loop function with arcpy.Exists()?

Question

I have a code snippet using elif that checks if a file exists and continues to check through the letters of the alphabet (for example _A, _B, _C etc). All works fine but I think there must a better (or cleaner) way to write the code using a loop function instead of the way I have done.

#Create MXD file path
mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01.mxd"
mxdPath = wrFolder + "/M_" + WorkrequestRaw +"_01"
#check for latest version and create next letter
if not arcpy.Exists(mxdOutput):
 mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01.mxd"
 pdfversion = ""
elif not arcpy.Exists(mxdPath +"_A.mxd"):
 mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_A.mxd"
 pdfversion = "A"
elif not arcpy.Exists(mxdPath +"_B.mxd"):
 mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_B.mxd"
 pdfversion = "B"
elif not arcpy.Exists(mxdPath +"_C.mxd"):
 mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_C.mxd"
 pdfversion = "C"
elif not arcpy.Exists(mxdPath +"_D.mxd"):
 mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_D.mxd"
 pdfversion = "D"

I am a self taught beginner and trying to keep the code tidy. Welcome any comments on a better methodology?

Update poorly written question: What I am trying to do is if file name exists write new file called 'filenameA' and if 'filenameA' exists write 'filenameB' and if 'filenameB exists write 'filenameC' etc.

Answer 1

names = ['','_A', '_B', '_C']
mxdPath = wrFolder + "/M_" + WorkrequestRaw +"_01"
for name in names:
 mxdOutput = mxdPath + name + '.mxd'
 if not arcpy.Exists(mxdOutput):
 pdfversion = name
 break

• mr.adam: nice job with putting '' in the list of names, hadn't thought of that.
• Michael Stimson: So what happens if A and B exist? In this code B trumps A.. that is probably the most likely situation. Mac, if you want to stop at A (or the earliest version that exists) you can include a break statement to stop the iteration. Nice solution Nathan, concise and simple.
• Mac Maclean: Thanks @Nathan W. I substituted the code and it wrote 'filenameC' first (instead of just 'filename', and then on the second iteration did nothing. I have edited my question to make it clearer
• Nathan W: Are you sure? It works for me here. i.imgur.com/qIwzn4r.png
• Nathan W: Well if you get that error you code isn't nested right. I suspect you don't have the if inside the for

Answer 2

This question is pretty close to off-topic (pure python). However, here is another method.

import os, sys, string
import arcpy
mxdPath = os.path.join(wrFolder, "M_" + WorkrequestRaw +"_01")
for letter in [''] + list(string.ascii_uppercase):
 mxdOutput = (mxdPath + '_' + letter).strip('_')+ '.mxd'
 if not arcpy.Exists(mxdOutput):
 pdfversion = name
 break
 if letter=='Z':
 raise RuntimeError('No more backup filenames available!')

• Michael Stimson: Good use of ascii_uppercase as a list, +1 for that. Doesn't account for '' though. Perhaps chars = [''].extend(string.ascii_uppercase) then for letter in chars.
• Nathan W: Didn't even know about ascii_uppercase. Nifty.
• Mac Maclean: This worked as well but as @Michael points out I need to check for '' first.

Answer 3

I strongly recommend using os.path.exists() when checking the existence of files (especially inside a loop)! The arcpy.Exists() function is slow and it is much faster to use os.path.exists().

Answer 4

Try this to construct name:

for i in range(10):
 fName='%s_%s.mxd'%("BASENAME",chr(65+i))

and break if does not exist. Construct new name using i

• Michael Stimson: That's a great method if letters go beyond C, that way you can go from A to Z and with a little more work with divide and mod you can do AA to ZZ. Doesn't account for '' though.