You're unlikely to get sufficiently fine-grained control that you can completely eliminate the possibility of duplicate timestamps - you'd need resolution smaller than the time it takes to generate a datetime object. There are a couple of other approaches you might take to deal with it:

1. Deal with it. Leave your timestamps non-unique as they are, but rely on python's sort being stable to deal with reordering problems. Sorting on timestamp first, then something else will retain the timestamp ordering - you just have to be careful to always start from the timestamp ordered list every time, rather than doing multiple sorts on the same list.

2. Append your own value to enforce uniqueness. Eg. include a monotonically incrementing integer as part of the key, or append such a value only if timestamps are different. Eg.

The following will guarantee unique timestamp values:

 class TimeStamper(object): def __init__(self): self.lock = threading.Lock() self.prev = None self.count = 0 def getTimestamp(self): with self.lock: ts = str(datetime.now()) if ts == self.prev: ts +='.%04d' % self.count self.count += 1 else: self.prev = ts self.count = 1 return ts 

For multiple processes (rather than threads), it gets a bit trickier though.

You're unlikely to get sufficiently fine-grained control that you can completely eliminate the possibility of duplicate timestamps - you'd need resolution smaller than the time it takes to generate a datetime object. There are a couple of other approaches you might take to deal with it:

1. Deal with it. Leave your timestamps non-unique as they are, but rely on python's sort being stable to deal with reordering problems. Sorting on timestamp first, then something else will retain the timestamp ordering - you just have to be careful to always start from the timestamp ordered list every time, rather than doing multiple sorts on the same list.

2. Append your own value to enforce uniqueness. Eg. include an incrementing integer value as part of the key, or append such a value only if timestamps are different. Eg.

The following will guarantee unique timestamp values:

 class TimeStamper(object): def __init__(self): self.lock = threading.Lock() self.prev = None self.count = 0 def getTimestamp(self): with self.lock: ts = str(datetime.now()) if ts == self.prev: ts +='.%04d' % self.count self.count += 1 else: self.prev = ts self.count = 1 return ts 

For multiple processes (rather than threads), it gets a bit trickier though.