Strange characters in output, even with colors disabled

[juanique]

I'm writing the output of of my tests to a log file that is visualized on a website. The UI does not support color so I'm running:

pytest ... --color=no > /tmp/test.log

If I cat /tmp/test.log the output looks OK.

self = <path.to.my.TestCase object at 0x7f6650312940>

    def foo(self):
      # ...
    
>       assert bar == {
            "anId": "a53ffeb6-99b0-4a03-8fb3-278c2559efad",
            "anotherId": "9d7a0b13-c92e-45f2-abc4-f260087516a7",
        }
E       AssertionError: assert equals failed
E         {                                                                        {                                                                       
E         -  'anId': 'e7dcba1a-d545-437c-8947-9d901729b51a',                    +  'anId': 'a53ffeb6-99b0-4a03-8fb3-278c2559efad',                   
E         -  'anotherId': '34f4eeab-1f1b-4d41-8421-d87b80000f8f',               +  'anotherId': '9d7a0b13-c92e-45f2-abc4-f260087516a7',             
E         }                                                                        }

There's no colors on the output, which is expected.

But in my Web UI these strange #x00 and #x01 characters appear:

self = <path.to.my.TestCase object at 0x7f6650312940>

    def foo(self):
      # ...
    
>       assert bar == {
            "anId": "a53ffeb6-99b0-4a03-8fb3-278c2559efad",
            "anotherId": "9d7a0b13-c92e-45f2-abc4-f260087516a7",
        }
E       AssertionError: assert equals failed
E         {                                                                        {                                                                       
E         #x00-  'anId': 'e7dcba1a-d545-437c-8947-9d901729b51a',#x01                    #x00+  'anId': 'a53ffeb6-99b0-4a03-8fb3-278c2559efad,#x01
E         #x00-  'anotherId': '34f4eeab-1f1b-4d41-8421-d87b80000f8f',#x01               #x00+  'anotherId': '9d7a0b13-c92e-45f2-abc4-f260087516a7',#x01
E         }   

Also if I cat -A instead of just cat I can see some of these invisible characters, which show up as ^A and ^@

self = <path.to.my.TestCase object at 0x7f6650312940>

    def foo(self):
      # ...
    
>       assert bar == {
            "anId": "a53ffeb6-99b0-4a03-8fb3-278c2559efad",
            "anotherId": "9d7a0b13-c92e-45f2-abc4-f260087516a7",
        }
E       AssertionError: assert equals failed
E         {                                                                        {                                                                       
E         ^@-  'anId': 'e7dcba1a-d545-437c-8947-9d901729b51a',^A                    ^@+  'anId': 'a53ffeb6-99b0-4a03-8fb3-278c2559efad',^A
E         ^@-  'anotherId': '34f4eeab-1f1b-4d41-8421-d87b80000f8f',^A               ^@+  'anotherId': '9d7a0b13-c92e-45f2-abc4-f260087516a7',^A
E         }   

What are these? how can I get rid of them?