Python FTW: Under the Hood/

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Let's Make a Giant String!

Let's Make a Giant String!

We'll cover the following...

How long does it take to build a giant string? Let’s find out.

Python 3.5
def add_string_with_plus(iters):
    s = ""
    for i in range(iters):
        s += "xyz"
    assert len(s) == 3*iters
def add_bytes_with_plus(iters):
    s = b""
    for i in range(iters):
        s += b"xyz"
    assert len(s) == 3*iters
def add_string_with_format(iters):
    fs = "{}"*iters
    s = fs.format(*(["xyz"]*iters))
    assert len(s) == 3*iters
def add_string_with_join(iters):
    l = []
    for i in range(iters):
        l.append("xyz")
    s = "".join(l)
    assert len(s) == 3*iters
def convert_list_to_string(l, iters):
    s = "".join(l)
    assert len(s) == 3*iters

Let’s use 1000 iterations to build a string.

The time is given in seconds.

Python 3.5
import timeit
NUM_ITERS = 1000
print(timeit.timeit('add_string_with_plus(NUM_ITERS)', number=1000, globals=globals()))
print(timeit.timeit('add_bytes_with_plus(NUM_ITERS)', number=1000, globals=globals()))
print(timeit.timeit('add_string_with_format(NUM_ITERS)', number=1000, globals=globals()))
print(timeit.timeit('add_string_with_join(NUM_ITERS)', number=1000, globals=globals()))
l = ["xyz"]*NUM_ITERS
print(timeit.timeit('convert_list_to_string(l, NUM_ITERS)', number=1000, globals=globals()))

Let’s increase the number of iterations by a factor of 10.

Python 3.5
import timeit
NUM_ITERS = 10000
print(timeit.timeit('add_string_with_plus(NUM_ITERS)', number=1000, globals=globals()))
print(timeit.timeit('add_bytes_with_plus(NUM_ITERS)', number=1000, globals=globals()))
print(timeit.timeit('add_string_with_format(NUM_ITERS)', number=1000, globals=globals()))
print(timeit.timeit('add_string_with_join(NUM_ITERS)', number=1000, globals=globals()))
l = ["xyz"]*NUM_ITERS
print(timeit.timeit('convert_list_to_string(l, NUM_ITERS)', number=1000, globals=globals()))

Explanation

  • You can read more about timeit or %timeit through these links. They are used to measure the execution time of code pieces.
  • Don’t use + for generating long strings—in Python, str is immutable, so the left and right strings have to be copied into the new string for every pair of concentrations.

  • If you concatenate four strings of length 1010, the number of ...