io

In-memory I/O streams. Currently provides StringIO — an in-memory string buffer that behaves like a text file.

Available Classes

Class	Description
`StringIO([initial_value])`	In-memory string buffer

StringIO

io.StringIO([initial_value=""]) creates an in-memory text buffer. An optional string pre-fills the buffer.

Methods

Method	Description
`write(s)`	Write string `s` to the buffer; returns number of characters written
`getvalue()`	Return the entire buffer contents as a string
`read([n])`	Read up to `n` characters from the current position; reads all if `n` is omitted
`readline()`	Read one line (including the newline character) from the current position
`seek(pos)`	Set the read position to `pos`
`tell()`	Return the current read position
`truncate([pos])`	Truncate the buffer to `pos` characters (default: current position)
`close()`	Close the buffer; further I/O raises an error

StringIO supports the with statement — the buffer is closed automatically on exit.

Usage Examples

Building strings incrementally

    
    
  
import io

buf = io.StringIO()
for i in range(5):
    buf.write(f"item {i}\n")

result = buf.getvalue()
print(result)
# item 0
# item 1
# item 2
# item 3
# item 4

Capturing print output

    
import io

buf = io.StringIO()
print("hello", file=buf)
print("world", file=buf)
captured = buf.getvalue()  # "hello\nworld\n"

Using as a context manager

    
import io

with io.StringIO() as buf:
    buf.write("line one\n")
    buf.write("line two\n")
    content = buf.getvalue()

# buf is closed after the with block
print(content)

Pre-filled buffer with read/seek

    
import io

buf = io.StringIO("hello world")

first = buf.read(5)   # "hello"
buf.seek(6)
rest  = buf.read()    # "world"

buf.seek(0)
all   = buf.read()    # "hello world"

Reading line by line

    
    
  
import io

buf = io.StringIO("line1\nline2\nline3")
while True:
    line = buf.readline()
    if line == "":
        break
    print(line.strip())

Truncating a buffer

    
import io

buf = io.StringIO("hello world")
buf.truncate(5)
print(buf.getvalue())  # "hello"

print(file=buf)

The built-in print() function accepts a file keyword argument. Passing a StringIO instance redirects output into the buffer instead of stdout.

    
import io

buf = io.StringIO()
print("a", "b", "c", sep=",", end="!\n", file=buf)
print(buf.getvalue())  # a,b,c!

Error Handling

Operations on a closed buffer raise an exception:

    
import io

buf = io.StringIO("data")
buf.close()

try:
    buf.write("more")
except Exception as e:
    print(e)  # I/O operation on closed file

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