Python StringIO ExamplesUse the StringIO class from the io namespace. StringIO writes to strings like files.
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StringIO writes to strings. This Python type uses a syntax similar to that for files. We can use print method calls, or invoke the write method. StringIO has performance advantages for creating large strings.
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. This is just for the example.
Write We call the write method to output data to our stream. It is just stored in memory. A print() call also works.
Getvalue The getvalue method returns a string containing the data written to the in-memory buffer.
Close We call the close method to prevent further writes to our StringIO stream. This is good housekeeping.
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 is powerful: it can target any compatible IO stream.
Console, print
Python program that uses StringIO
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
Performance. Is StringIO faster than a series of string appends? I tested a program that wrote large strings, appending strings many times. I found StringIO was faster, in two Python implementations.
PyPy In the PyPy implementation, 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.
Python program that uses print
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())
Summary. Many ways exist to append strings together. StringIO is faster than using direct string concats on large strings. It introduces syntactic complexity, though: strings alone are simpler.
Python program that benchmarks StringIO, string concats
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
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