Python prints messages not sequentially

Question

if I have this code

def soft_assert(condition):
    if not condition:
        print('start')
        traceback.print_stack()
        print('finish')

soft_assert(False)
soft_assert(False)

the output is different every time I run it:

  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 38, in <module>
    soft_assert(False, 'message 1')
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 30, in soft_assert
    traceback.print_stack()
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 41, in <module>
    soft_assert(False, 'message 4')
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 30, in soft_assert
    traceback.print_stack()
start
finish
start
finish

or

start
finish
start
finish
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 38, in <module>
    soft_assert(False)
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 30, in soft_assert
    traceback.print_stack()
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 41, in <module>
    soft_assert(False)
  File "/Users/spleshakov/Documents/api-testing-with-pytest/tenants/test_uc.py", line 30, in soft_assert
    traceback.print_stack()

The output can be in any order you can think of

This doesn't make sense to me, I can't find an answer, so I was hoping the community can help me with these questions:

1. Why this is happening?
2. How do I make wait one print to finish, before doing another one?

Thanks

Answer 1

The problem is it prints by default to the stderr, not the stdout:

# Used by print_stack
def print_list(extracted_list, file=None):
    if file is None:  # Note here
        file = sys.stderr
    for item in StackSummary.from_list(extracted_list).format():
        print(item, file=file, end="")

You're getting out-of-order output because the two streams are being printed to the same console at once.

Specify the file argument so it all gets printed to the same stream:

from sys import stdout

def soft_assert(condition):
    if not condition:
        print('start')
        traceback.print_stack(file=stdout)  # Print to stdout
        print('finish')