mpfs: MicroPython File System

Rationale

The MicroPython 1.9.4 implementation for Casio calculators does not currently provide a working open() call nor os module. The mpfs module works around those limitations by mounting a transparently-compressed read-only file system that is accessible to user scripts. It provides the most useful methods of the file object and equivalents for several functions in os and os.path.

Installation

Run sudo ./install.sh to install mkmpfs.

Next, copy the mpfs.py module to the Casio calculator's USB storage.

To uninstall, run sudo ./uninstall.sh.

Build and install a file system

The mkmpfs program will compress the source files and build a file system. For example, to create a Python module named test with a file system containing the data from files a.txt and data/b.dat, run:

mkmpfs a.txt data/b.dat >test.py

The file system does not support a directory structure. In the above command, data is read from data/b.dat, however the file will be named b.dat inside the file system.

To install the file system module, copy test.py to the Casio calculator's USB storage.

Import mpfs

The recommended way to import the mpfs module is:

import mpfs;from mpfs import open

Mount a file system

If the file system is named test.py then the Python module name will be test. Mount the file system with this command:

mpfs.mount("test")

Although mount() may be called multiple times, only one file system may be mounted at a time. Each subsequent call replaces the prior file system.

Remounting a file system will not restore removed files; restart the program instead.

File operations

Read a text file

>>> with open("a.txt") as f:
...     for line in f:
...         print(line, end="")
...
Abc
defgh
ijk.

Other file methods to read lines from a file are readline() and readlines():

>>> with open("a.txt") as f:
...     f.readline()
...     f.readlines()
...
'Abc\n'
['defgh\n', 'ijk.\n']

More information:

• https://docs.python.org/3/library/functions.html#open
• https://docs.python.org/3/library/io.html#io.IOBase
• https://docs.python.org/3/library/io.html#io.IOBase.readline
• https://docs.python.org/3/library/io.html#io.IOBase.readlines

Read a binary file

>>> with open("b.dat") as f:
...     data = f.read(10)
...
>>> data
b'\x02\x80@\x80?\xe0\x0f\xf8\x03\xfe'

Rather than reading 10 bytes, the entire file could have been be read at once using f.read(). However, with large files there may be insufficient memory to do so. Instead, read and process fewer bytes at a time. Depending on the situation, it may be helpful to make use of Python's yield keyword.

More information:

• https://docs.python.org/3/library/functions.html#open
• https://docs.python.org/3/library/io.html#io.IOBase
• https://docs.python.org/3/library/io.html#io.RawIOBase.read

Get or set current file position

>>> with open("a.txt") as f:
...     f.readline()
...     f.tell()
...     f.seek(0)
...     f.readline()
...     f.seek(-3, mpfs.SEEK_END)
...     f.read()
...
'Abc\n'
4
0
'Abc\n'
12
b'k.\n'

More information:

• https://docs.python.org/3/library/io.html#io.IOBase.seek
• https://docs.python.org/3/library/io.html#io.IOBase.tell

File system information

List files

>>> mpfs.listdir("")
['a.txt', 'b.dat']

More information:

• https://docs.python.org/3/library/os.html#os.listdir

Get uncompressed file size, in bytes

>>> mpfs.getsize("a.txt")
15

More information:

• https://docs.python.org/3/library/os.path.html#os.path.getsize

Determine whether a file exists

>>> mpfs.exists("c.txt")
False

More information:

• https://docs.python.org/3/library/os.path.html#os.path.exists

Delete a file

Although the file system is read-only, when a file system is mounted it (and all the files it contains) will be copied to Python's heap, taking up space. If a file is no longer needed, it can be removed to free up some heap space. The files are not permanently deleted and will return the next time the program is run.

>>> mpfs.listdir("")
['a.txt', 'b.dat']
>>> mpfs.remove("b.dat")
>>> mpfs.listdir("")
['a.txt']

More information:

• https://docs.python.org/3/library/os.html#os.remove

Indices

Internal functions and builtin replacements

• mpfs.mount(module: str) -> None
• open(filename: str) -> mpfs.Mpfs

More information:

• https://docs.python.org/3/library/functions.html#open

Mpfs file object methods

• close() -> None
• read(n: int=-1) -> bytes()
• readline() -> str
• readlines() -> list[str]
• seek(offset: int, whence: int=mpfs.SEEK_SET) -> int
• seek(offset: int, mpfs.SEEK_CUR) -> int
• seek(offset: int, mpfs.SEEK_END) -> int
• tell() -> int

More information:

• https://docs.python.org/3/library/io.html#io.IOBase
• https://docs.python.org/3/library/io.html#io.RawIOBase

Function equivalents for the os module

• mpfs.listdir(_) -> list[str]
• mpfs.remove(filename: str) -> None

More information:

• https://docs.python.org/3/library/os.html#files-and-directories

Function equivalents for the os.path module

• mpfs.exists(filename) -> bool
• mpfs.getsize(filename) -> int

More information:

• https://docs.python.org/3/library/os.path.html#module-os.path