Appending to Long Strings in Python

Be careful when repeatedly appending to a string in Python. Because strings are immutable, each time you change one, a new object is created. The code below is very intuitive:


s = ''
for i in range(r):
    s += str(i)

But it doesn’t perform too well.

A better approach is to create and append to a list and then convert it to a string using ''.join(l), like this:


l = []
for i in range(r):
s =''.join(l)

And an even slightly faster approach is to use join() with a list comprehension:


s = ''.join([str(i) for i in range(r)])

Here are the three methods compared with timeit:Methods for concatenating strings

As you can see, the latter two methods, which modify a list and then convert it to a string, run 4 to 5 times as quickly. The larger the string (i.e., the bigger the object), the bigger the difference will be.

Related Training: Advanced Python

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