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cs2370 Notes: 15 Files and Directories

··4 mins

Files and Directories

Modern computers have mostly settled on a single mechanism to store persistent data, or data that survives when you close your applications and shut off the computer: Files and Directories.

This is true for Windows, Mac, Linux, ChromeOS, iOS, and Android, although some systems try to hide this stuff from users more than others do.

Files

We keep data in files. A file is:

  • A series of 0 or more bytes of data.
  • Some associated metadata, like a file name.

Directories

To keep files organized, each file goes in a directory.

Directories can also store other directories, so we end up with a “tree” structure.

On Windows:

  • Open up the Windows file manager.
  • Navigate to the root.
  • Windows separates different “drives”, so each drive is its own directory tree. Drives are identifed by drive letter.
  • C:\ is the root directory of the main drive.
  • It contains several other files and directories.
  • Let’s draw a subtree of this out on the board.

On Linux:

  • Open up Nemo
  • Start at /
  • UNIX family systems like Linux and Mac don’t separate different drives into different trees - there’s one tree and the root is ‘/’.
  • Click around a bit.
  • Let’s draw a subtree of this out on the board.

Paths

Every file and directory on a computer has a unique path. This is a string that lists the directories you need to traverse from the root to get to that file/directory. On windows, it starts with a drive letter.

For example, the Windows path C:\Users\nt1171\IP\walrus.py tells us:

  • This path is on the C drive.
  • Inside the root directory is “Users”
  • Inside that directory is “nt1171”
  • Then “IP”
  • Then “walrus.py”

Probably “walrus.py” is a file, but there’s no way by just looking at the path to determine that it’s not a directory.

Probalby “walrus.py” is a Python source file, but the “.py” is just part of the file name that conventionaly indicates file type. This could be a JPEG image that someone decided to save with that name.

Windows tree demo: On Windows, try C:\> tree /f /a.

The Linux path ````/home/nat/Teaching/cs2370/syllabus.pdf``` contains a “home” directory in the root directory, a series of other directories, and a final part “syllabus.pdf”. Again, this is probably a PDF file, but we can’t be sure by looing at the path.

Note that Windows and UNIX-type systems use different characters as path separators. On Windows it’s backslash, on UNIX it’s forward slash. Forward slash is more convenient in programming - you don’t need to escape it in strings - but using backslash means that file names on Windows can have forward slashes in them.

Those path strings uniquely identify a file or directory on a computer, but there are also path strings that identify a path relative to the current working directory.

Every program on a computer is running in some specific directory. It can access files in that directory by simply specifying their filename. It also can do other relative paths:

  • “./foo” is a thing called “foo” in the current directory
  • “../foo” is a thing called “foo” in the parent directory, or the directory that contains the current directory.

When writing cross-platform code, it can be useful to generate different path strings on different platforms to get the correct directory separators (and possibly drive letters).

Try this on Windows and on Linux:

>>> from pathlib import Path
>>> xx = Path("/foo/bar/baz")
>>> xx
>>> str(xx)
>>> Path("/") / "tmp" / "walrus.dat"

On Linux, the path object is called a PosixPath. “POSIX” - portable operating system interface - is a standard that describes the common properties of all modern UNIX-style operating sytems, including how the file system works.

More stuff in path

>>> Path.cwd()
>>> Path.home()
>>> Path.abspath('.')
>>> Path.abspath('.').glob("*.py")

Path methods:

  • exists()
  • is_file()
  • is_dir()
  • read_text()
  • write_text()

What else to do with a path:

  • open

File Handles

The open bif gives us a file object. By default files are opened in text mode to read a series of characters, but they also can be opened in binary mode to read a series of bytes.

Methods (on text file):

  • close - When we’re done; open files are a limited resource.
  • readline - Read one line
  • write - write a string to the file
  • read - read the whole file

Do some examples, including writing to file.