heroku local webでローカルで起動したflaskアプリのprint()の出力が表示されない。

Question

heroku local web -f Procfile.windowsでローカルにあるpythonアプリ(app.py)を起動したのですがprint()の出力の一部がコマンドプロンプトに表示されません。出力結果はどこで見れるのでしょうか？

from flask import Flask

app = Flask(__name__)

print("hoge1") #表示される

@app.route('/')
def index():
    print("hoge2") #表示されない
    x=1/0 #エラーの詳細は表示される(ZeroDivisionError: division by zero)
    return ""

if __name__ == '__main__':
    app.run()

Answer 1

コマンドプロンプト(cmd.exe)から実行すると、表示されると思います。

Pythonは標準出力(printの出力先)が仮想端末に繋がっていないとバッファリングされてしまいます。
(すみません。私のPCはLinuxです。)

端末から直接実行 -> hoge2が出力される。

$ python app.py
hoge1
 * Serving Flask app "app" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
hoge2
[2021-04-04 18:01:55,652] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/ysk/work/teratail.com/329656/app.py", line 10, in index
    x=1/0 #エラーの詳細は表示される(ZeroDivisionError: division by zero)
ZeroDivisionError: division by zero
127.0.0.1 - - [04/Apr/2021 18:01:55] "GET / HTTP/1.1" 500 -

仮想端末から切り離して実行(catコマンドにパイプ) -> hoge2が出力されない

$ python app.py |cat
hoge1
 * Serving Flask app "app" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
[2021-04-04 18:03:55,238] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/ysk/.local/share/virtualenvs/329656-tzNgrqId/lib/python3.9/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/ysk/work/teratail.com/329656/app.py", line 10, in index
    x=1/0 #エラーの詳細は表示される(ZeroDivisionError: division by zero)
ZeroDivisionError: division by zero
127.0.0.1 - - [04/Apr/2021 18:03:55] "GET / HTTP/1.1" 500 -

なお、別の方法としてprintへflush=Trueを追加すると出力されると思います。

...
    print("hoge2", flush=True) #表示されない
...