`StringIO`

This Python type writes to strings. It uses a syntax similar to that for files. We can use print method calls, or invoke the write method.

Performance notes

StringIO has performance advantages for creating large strings. In some implementations, programs may run twice as fast.

We first import the io namespace

We create a new stream by calling io.StringIO: this returns a stream variable. Next, in a for-loop, I write some data to our stream.

Info We call the write method to output data to our stream. It is just stored in memory. A print() call also works.

Tip The getvalue method returns a string containing the data written to the in-memory buffer.

Finally We call the close method to prevent further writes to our StringIO stream. This is good housekeeping.

import io

out = io.StringIO()

# Print these string values in a loop.
for i in range(0, 100):
    out.write("Value = ")
    out.write(str(i))
    out.write(" ")

# Get string and display first 20 character.
data = out.getvalue()
print(data[0:20])

out.close()
Value = 0 Value = 1

Print

We do not need to use write() to write with StringIO. We can instead use print() and specify the optional "file" argument. The print method can target any compatible IO stream.

import io

out = io.StringIO()

# Print to StringIO stream, no end char.
print("Hello", file=out, end="")

# Print string contents to console.
print(out.getvalue())
Hello

Performance

Is StringIO faster than a series of string appends? I tested a program that wrote large strings, appending strings many times.

Result I found StringIO was faster, in two Python implementations. In PyPy, StringIO was nearly twice as fast as a string append.

Note The if-check in the benchmark loops is just a simple sanity check. The strings must begin with the letter V.

import io
import time

print(time.time())

# 1. Use StringIO.
for x in range(0, 10000):
    out = io.StringIO()
    for i in range(0, 100):
        out.write("Value = ")
        out.write(str(i))
        out.write(" ")
    # Get string.
    contents = out.getvalue()
    out.close()
    # Test first letter.
    if contents[0] != 'V':
        raise Error

print(time.time())

# 2. Use string appends.
for x in range(0, 10000):
    data = ""
    for i in range(0, 100):
        data += "Value = "
        data += str(i)
        data += " "
    # Test first letter.
    if data[0] != 'V':
        raise Error

print(time.time())1404346870.027      [PyPy3 2.3.1]
1404346870.286      StringIO: 0.259 s
1404346870.773      Concat:   0.487 s

1404346852.222672   [Python 3.3]
1404346853.343738   StringIO: 1.121 s
1404346854.814824   Concat:   1.471 s

Summary

Many ways exist to append strings together. StringIO is faster than using direct string concats on large strings. It has more complex syntax than string appends.