How deeply does The Quick Python Book, Fourth Edition cover input/output redirection?

Explains shell operators and Python stream handling, then develops an optional-filename pattern for a word-count utility.

Input/output redirection

As explained in The Quick Python Book, Fourth Edition

Input redirection is a shell mechanism that supplies a command's standard input from a file or another source instead of an interactive terminal.

Input/output redirection changes where a command reads or writes without changing its core job. At a command prompt, `<` supplies standard input from a file, while `>` sends standard output to a file. This makes it possible to connect small, specialized programs. section 11.1.4

Replace text from one file into another

`python replace.py zero 0 < infile > outfile` reads `infile`, replaces occurrences of `zero` with `0`, and writes the result to `outfile`. The two words after the script name remain command-line arguments; the redirected file is not an argument. section 11.1.4

Listing 11.3 File replace.py

import sys
def main():
   contents = sys.stdin.read()                                  #1
   sys.stdout.write(contents.replace(sys.argv[1], sys.argv[2])) #2
if __name__ == "__main__":
  main()

The script reads `sys.stdin` to end-of-file and writes its transformed contents to `sys.stdout`. `print()` also writes to standard output by default. section 11.1.4

The same streams support a few useful variations:

Use `>>` instead of `>` to append output rather than overwrite the destination file. section 11.1.4
Use a pipe (`|`) to feed one command’s output into another command’s input. section 11.1.4
Without redirected input, a script reading `sys.stdin` waits for terminal input until end-of-file; on the described Unix-style terminal, Ctrl-D signals it. section 11.1.4

A file-processing command can accept either a filename or redirected input. For a `wc`-style utility, make the filename optional and read `sys.stdin` when it is absent. `argparse` can do this with `nargs='?'`, `type=argparse.FileType('r')`, and `default=sys.stdin`; the counting code can then process either source as a file object. section 13.10 section 13.10.2

Redirection can also happen inside Python: assign a file object to `sys.stdout` to capture subsequent output, then restore the previous stream and close the file. For a single `print()` call, its `file` parameter is simpler. In Colaboratory or Jupyter, save a redirection-dependent script as a `.py` file and run it with `! python ...`; restoring a stream there requires saving its previous value rather than relying on `sys.__stdout__`. section 13.6 section 13.10.2

How The Quick Python Book, Fourth Edition covers it

Cover of The Quick Python Book, Fourth Edition

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Related concepts

wc utilityused by · The wc utility is a Unix command-line tool that reports counts of lines, words, bytes, characters, and related properties in files or input streams.
file objectuses · A file object is a program-accessible representation of an open file or stream, including its access state and input/output operations.
standard inputuses · Standard input is the conventional input stream through which a process receives data from its execution environment or an attached source.
pipelineimplemented by · A pipeline is an ordered or coordinated sequence of connected processing stages or tasks that passes data, work, or results toward an outcome.
command-line interfaceused by · A command-line interface is a text-based interface through which users or programs interact with software or a computer by entering commands and receiving output.
end-of-file indicatoruses · An end-of-file indicator is a condition, signal, or convention indicating that no more input data is available from a file or stream.

This explainer was generated by Manning's knowledge graph from the text of The Quick Python Book, Fourth Edition. The section links point to the parts of the book it draws on.