Pick up to N keys that match a pattern on a large Redis instance

redis-scan-first-n-matches.py

import itertools

def scan(c, pattern, n):
  """Find up to N Redis keys that match a pattern.
  
  NOTE: even if this Python code looks innocuous, it involves
        A LOT of work on the Redis instance, especially when
        the number of matches is low with respect to the total
        number of keys.  Use with care!
  """
  
  # SCAN can return duplicates.
  keys = set()
  
  cursor, results = c.scan(0, pattern)
  while cursor != 0 and len(keys) < n:
  
    # Fill up to K remaining slots.
    keys = keys.union(itertools.islice(results, n - len(keys)))
    
    cursor, results = c.scan(cursor, pattern)
  return keys