Getting rid of old string values

[Pedroski55]

Recently I posted something here, can't remember exactly which thread, in which I started with an empty string and kept adding bits to it.

deanhystad pointed out, because strings are immutable, I would end up with a lot of strings hanging around in memory. Python doesn't overwrite them. I don't know what happens to them or how Python deals with them.

I was thinking of that today. I get a date from Excel, like 2022-05-21 04:33:27 and put it in this function:

def string2Seconds(date):
    st = str(date)
    # need to specify the format of the string
    dt = datetime.strptime(st, "%Y-%m-%d %H:%M:%S")
    # convert dt to epoch seconds
    seconds = dt.timestamp()
    return seconds

Maybe I am looking for all rows in the Excel after 2022-05-21 04:33:27 or before 2022-05-21 04:33:27

That would leave a lot of old st variables hanging around in memory.

Is there anything I can do about that??

[Gribouillis]

I don't understand the problem. The st variables are local variables. They are returned to the garbage collector when the function exits. They wont "hang in memory".

[perfringo]

Carbage collector works based on reference count. So non-referenced objects (of string) will be destroyed.

The real problem with concatenating strings is it's quadratic runtime cost. From Python documentation:

Concatenating immutable sequences always results in a new object. This means that building up a sequence by repeated concatenation will have a quadratic runtime cost in the total sequence length. To get a linear runtime cost, you must switch to one of the alternatives below:

• if concatenating str objects, you can build a list and use str.join() at the end or else write to an io.StringIO instance and retrieve its value when complete
  
• if concatenating bytes objects, you can similarly use bytes.join() or io.BytesIO, or you can do in-place concatenation with a bytearray object. bytearray objects are mutable and have an efficient overallocation mechanism
  
• if concatenating tuple objects, extend a list instead
  
• for other types, investigate the relevant class documentation
  

[Pedroski55]

Thanks for your replies.

OK, I obviously misunderstood deanhystad.

Unreferenced variables will be trashed when a function exits. Use lists if you can.

Got it!