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Adapted and simplified from Franck Albinet's exercises available here. Also presented by NSRC at APRICOT 2018.

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While one of the primary purpose of IoT is to collect and exchange data over an inter-connected network, it is as well important to be able to persist information in the IoT device itself: log files of device's activity, Received Signal Strength Activity (RSSI), ... In the case of high-resolution measurements, it's useful to store them before sending them over the network, in order to save on power.

Learning outcomes

You will learn how to: access and operate device's file system; create and write a file in the flash folder; handle files safely using with statement; generate programmatically time stamped log file names; make your code robust and flexible with try ... except statement.

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Import the basic operating system module (os): import os., then try a few commands:

import os
# to find you current working directory:
os.getcwd() ## most probably the /flashfolder
# to list folders and files in your current working directory:
os.listdir()
# to create a new folder/directory named "log":
os.mkdir('log'); ...

Take a look at os module documentation for a full list of methods.

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For further reference on reading and writing files in Python, look at the official documentation here.

But in In essence to handle files in Python, you first need to open a file (even if it does not exist yet)

python f = open('log/my_first_file.log', 'w')

the open function takes as argument: file name 'log/my_first_file.log' (relative or full path) and mode: read, write, ...

Once open, you get a file object to play with and hence can start writing data in it: 

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Then you need to close the file to free up any system resources taken up by the open file. After calling

f.close()

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attempts to use the file object will automatically fail.

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Finally, before creating a folder or a file, we would like to test if it exists already. The code below test it and recap. the whole process:{code

import os
file_path = '/flash/log'

try:
    os.listdir('/flash/log')
    print('/flash/log file already exists.')
except OSError:
    print('/flash/log file does not exist. Creating it ...')
    os.mkdir('/flash/log')

name = '/my_first_file.log'

# Writing
with open(file_path + name, 'w') as f:
    f.write('Testing write operations in a file.')

# Reading
with open(file_path + name, 'r') as f:
    print(f.readall())

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