Python 不按顺序打印消息

Question

如果我有这个代码

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

soft_assert(False)
soft_assert(False)

每次运行输出都不一样:

  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

或

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()

输出可以是你能想到的任何顺序

这对我来说没有意义，我找不到答案，所以我希望社区可以帮助我解决这些问题:

1. 为什么会这样？
2. 如何让等待一个 print 完成，然后再执行另一个？

谢谢

Answer 1

问题是它默认打印到 stderr，而不是 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="")

你得到乱序的输出，因为这两个流被同时打印到同一个控制台。

指定 file 参数，使其全部打印到同一流:

from sys import stdout

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