How to Repeatedly Append to a String in Python
In Python, if you need to repeatedly append to a string, you should convert it to a list, append your items to that list, and then join the list back into a string after you've made all the additions.
To illustrate, imagine you are reading through a long log file looking for lines that begin with "ERROR:" You want to store all those lines in a string and write them out to a new file.
Your first inclination might be to do that like this:
with open('log.txt') as f: log = f.readlines() new_log = '' for line in log: if line[:6] == 'ERROR:': new_log += line with open('newlog.txt','w') as f: f.write(new_log)
But as Python strings are immutable, that method requires creating new memory allocation each time you modify the string.
Instead, you should do the following:
- Open and read the log file into a list (e.g.,
with open('log.txt') as f: log = f.readlines()
- Create a new empty list (e.g.,
new_log = 
- Loop through the
loglist looking for matches and append each matching line to the
for line in log: if line[:6] == 'ERROR:': new_log.append(line)
new_logon an empty string and write it to the new log file:
with open('newlog.txt','w') as f: f.write(''.join(new_log))
If you're only making a few changes to the string, you won't notice much difference, but if you're making thousands, this method is much more efficient.