exec() not returning process ID

SVN command line errors go to stderr, not stdout, which is why you aren't able to see them. What you want to do is redirect stderr to stdout and then print_r($output) to determine the cause of the error.

exec('svn commit <stuff> 2>&1', $output, $returnStatus);
if ( $returnStatus )
{
    print_r($output);
}

Sample output:

Array
(
    [0] => svn: Commit failed (details follow):
    [1] => svn: '/path/to/<stuff>' is not under version control
)

This is not ideal because it mixes stderr with stdout when it would otherwise be unnecessary, but I don't know of another way to get the desired output in this case. If someone else does, please comment.

Note: The third parameter to exec(...) is the return status, not the error message, so you need to tweak your code accordingly for that as well.

If the error output after making the 2>&1 modification doesn't help solve the root cause of your problem, please post the new output here.

Edit after new information:

Did you upload this SVN working copy to your server from somewhere else? That would explain this error.

If I check out a working copy from my local repository, upload it to my remote server, and then try to commit, I get the same error.

If that's the case, you need to execute the "svn checkout" of the working copy on the server running the PHP script in order to be able to commit to it. If that server can't communicate with the repository's server, then that's a whole other problem.

Perhaps a little shorter and faster way, since by all odds this function is going to be used on very large data:

from Bio import Seq
from itertools import product

def extend_ambiguous_dna(seq):
   """return list of all possible sequences given an ambiguous DNA input"""
   d = Seq.IUPAC.IUPACData.ambiguous_dna_values
   return [ list(map("".join, product(*map(d.get, seq)))) ]

Using map allows your loops to be executed in C rather than in Python. This should prove much faster than using plain loops or even list comprehensions.

Field testing

With a simple dict as d instead of the one returned by ambiguous_na_values

from itertools import product
import time

d = { "N": ["A", "G", "T", "C"], "R": ["C", "A", "T", "G"] }
seq = "RNRN"

# using list comprehensions
lst_start = time.time()
[ "".join(i) for i in product(*[ d[j] for j in seq ]) ]
lst_end = time.time()

# using map
map_start = time.time()
[ list(map("".join, product(*map(d.get, seq)))) ]
map_end = time.time()

lst_delay = (lst_end - lst_start) * 1000
map_delay = (map_end - map_start) * 1000

print("List delay: {} ms".format(round(lst_delay, 2)))
print("Map delay: {} ms".format(round(map_delay, 2)))

Outputs:

# len(seq) = 2:
List delay: 0.02 ms
Map delay: 0.01 ms

# len(seq) = 3:
List delay: 0.04 ms
Map delay: 0.02 ms

# len(seq) = 4
List delay: 0.08 ms
Map delay: 0.06 ms

# len(seq) = 5
List delay: 0.43 ms
Map delay: 0.17 ms

# len(seq) = 10
List delay: 126.68 ms
Map delay: 77.15 ms

# len(seq) = 12
List delay: 1887.53 ms
Map delay: 1320.49 ms

Clearly map is better, but just by a factor of 2 or 3. It's certain it could be further optimised.