This tutorial describes how MicroPython provides an on-device filesystem, allowing standard Python file I/O methods to be used with persistent storage.
MicroPython automatically creates a default configuration and auto-detects the primary filesystem, so this tutorial will be mostly useful if you want to modify the partitioning, filesystem type, or use custom block devices.
The filesystem is typically backed by internal flash memory on the device, but can also use external flash, RAM, or a custom block device.
On some ports (e.g. STM32), the filesystem may also be available over USB MSC to a host PC. The pyboard.py tool also provides a way for the host PC to access to the filesystem on all ports.
Note: This is mainly for use on bare-metal ports like STM32 and ESP32. On ports with an operating system (e.g. the Unix port) the filesystem is provided by the host OS.
MicroPython implements a Unix-like Virtual File System (VFS) layer. All mounted
filesystems are combined into a single virtual filesystem, starting at the root
/. Filesystems are mounted into directories in this structure, and at
startup the working directory is changed to where the primary filesystem is
On STM32 / Pyboard, the internal flash is mounted at
/flash, and optionally
the SDCard at
/sd. On ESP8266/ESP32, the primary filesystem is mounted at
/. On the OpenMV Cam the internal flash is mounted at
/ unless an SDCard
is installed which will be moutned at
A block device is an instance of a class that implements the
Ports provide built-in block devices to access their primary flash.
On power-on, MicroPython will attempt to detect the filesystem on the default flash and configure and mount it automatically. If no filesystem is found, MicroPython will attempt to create a FAT filesystem spanning the entire flash. Ports can also provide a mechanism to “factory reset” the primary flash, usually by some combination of button presses at power on.
The pyb.Flash class provides access to the internal flash. On some
boards which have larger external flash (e.g. Pyboard D), it will use that
start kwarg should always be specified, i.e.
Note: For backwards compatibility, when constructed with no arguments (i.e.
pyb.Flash()), it only implements the simple block interface and reflects the
virtual device presented to USB MSC (i.e. it includes a virtual partition table
at the start).
The internal flash is exposed as a block device object which is created in the
flashbdev module on start up. This object is by default added as a global
variable so it can usually be accessed simply as
bdev. This implements the
The following class implements a simple block device that stores its data in
RAM using a
class RAMBlockDev: def __init__(self, block_size, num_blocks): self.block_size = block_size self.data = bytearray(block_size * num_blocks) def readblocks(self, block_num, buf): for i in range(len(buf)): buf[i] = self.data[block_num * self.block_size + i] def writeblocks(self, block_num, buf): for i in range(len(buf)): self.data[block_num * self.block_size + i] = buf[i] def ioctl(self, op, arg): if op == 4: # get number of blocks return len(self.data) // self.block_size if op == 5: # get block size return self.block_size
It can be used as follows:
import os bdev = RAMBlockDev(512, 50) os.VfsFat.mkfs(bdev) os.mount(bdev, '/ramdisk')
An example of a block device that supports both the simple and extended
interface (i.e. both signatures and behaviours of the
uos.AbstractBlockDev.writeblocks() methods) is:
class RAMBlockDev: def __init__(self, block_size, num_blocks): self.block_size = block_size self.data = bytearray(block_size * num_blocks) def readblocks(self, block_num, buf, offset=0): addr = block_num * self.block_size + offset for i in range(len(buf)): buf[i] = self.data[addr + i] def writeblocks(self, block_num, buf, offset=None): if offset is None: # do erase, then write for i in range(len(buf) // self.block_size): self.ioctl(6, block_num + i) offset = 0 addr = block_num * self.block_size + offset for i in range(len(buf)): self.data[addr + i] = buf[i] def ioctl(self, op, arg): if op == 4: # block count return len(self.data) // self.block_size if op == 5: # block size return self.block_size if op == 6: # block erase return 0
As it supports the extended interface, it can be used with
import os bdev = RAMBlockDev(512, 50) os.VfsLfs2.mkfs(bdev) os.mount(bdev, '/ramdisk')
Once mounted, the filesystem (regardless of its type) can be used as it normally would be used from Python code, for example:
with open('/ramdisk/hello.txt', 'w') as f: f.write('Hello world') print(open('/ramdisk/hello.txt').read())
The following table shows which filesystems are included in the firmware by default for given port/board combinations, however they can be optionally enabled in a custom firmware build.
|Board||FAT||littlefs v1||littlefs v2|
|pyboard 1.0, 1.1, D||Yes||No||Yes|
The main advantage of the FAT filesystem is that it can be accessed over USB MSC on supported boards (e.g. STM32) without any additional drivers required on the host PC.
However, FAT is not tolerant to power failure during writes and this can lead to filesystem corruption. For applications that do not require USB MSC, it is recommended to use littlefs instead.
To format the entire flash using FAT:
# ESP8266 and ESP32 import os os.umount('/') os.VfsFat.mkfs(bdev) os.mount(bdev, '/') # STM32 import os, pyb os.umount('/flash') os.VfsFat.mkfs(pyb.Flash(start=0)) os.mount(pyb.Flash(start=0), '/flash') os.chdir('/flash')
Littlefs is a filesystem designed for flash-based devices, and is much more resistant to filesystem corruption.
Note: It can be still be accessed over USB MSC using the littlefs FUSE
driver. Note that you must use the
-b=4096 option to override the block
To format the entire flash using littlefs v2:
# ESP8266 and ESP32 import os os.umount('/') os.VfsLfs2.mkfs(bdev) os.mount(bdev, '/') # STM32 import os, pyb os.umount('/flash') os.VfsLfs2.mkfs(pyb.Flash(start=0)) os.mount(pyb.Flash(start=0), '/flash') os.chdir('/flash')
By using the
len kwargs to
pyb.Flash, you can create
block devices spanning a subset of the flash device.
For example, to configure the first 256kiB as FAT (and available over USB MSC), and the remainder as littlefs:
import os, pyb os.umount('/flash') p1 = pyb.Flash(start=0, len=256*1024) p2 = pyb.Flash(start=256*1024) os.VfsFat.mkfs(p1) os.VfsLfs2.mkfs(p2) os.mount(p1, '/flash') os.mount(p2, '/data') os.chdir('/flash')
This might be useful to make your Python files, configuration and other rarely-modified content available over USB MSC, but allowing for frequently changing application data to reside on littlefs with better resilience to power failure, etc.
The partition at offset
0 will be mounted automatically (and the filesystem
type automatically detected), but you can add:
import os, pyb p2 = pyb.Flash(start=256*1024) os.mount(p2, '/data')
boot.py to mount the data partition.
On ESP32, if you build custom firmware, you can modify
define an arbitrary partition layout.
At boot, the partition named “vfs” will be mounted at
/ by default, but any
additional partitions can be mounted in your
import esp32, os p = esp32.Partition.find(esp32.Partition.TYPE_DATA, label='foo') os.mount(p, '/foo')