Count number of files with certain extension in Python

Your code is fine.

Yes, you're going to need to loop over those files to filter out the .tif files, but looping over a small in-memory array is negligible compared to the work of scanning the file directory to find these files in the first place, which you have to do anyway.

I wouldn't worry about optimizing this code.

For this particular use case, if you don't want to recursively search in the subdirectory, you can use os.listdir:

len([f for f in os.listdir(myPath) 
     if f.endswith('.tif') and os.path.isfile(os.path.join(myPath, f))])

Something has to iterate over all files in the directory, and look at every single file name - whether that's your code or a library routine. So no matter what the specific solution, they will all have roughly the same cost.

If you think it's too much code, and if you don't actually need to search subdirectories recursively, you can use the glob module:

import glob
tifCounter = len(glob.glob1(myPath,"*.tif"))