Preface
Flask will output unstructured request logs in the console by default. If you want to output a json format log and write the request log to a separate file, you can do so by first disabling the default request log and then recording the requests in the hook function.
Define loggers
The following code defines two JSON log formatters.JsonFormatterThe log format is for use in ordinary code, and it will record information such as calling functions, calling files, etc.AccessLogFormatterThe log format is used to record request logs, and to record request path, response status code, response time and other information.
FlaskLoggerThrough inheritanceTo implement some custom functions, such as specifying a formatter, creating a log directory, etc.
class JsonFormatter():
def format(self, record: ):
log_record = {
"@timestamp": (record, "%Y-%m-%dT%H:%M:%S%z"), # format iso 8601
"level": ,
"name": ,
"file": ,
"lineno": ,
"func": ,
"message": (),
}
return (log_record)
class AccessLogFormatter():
def format(self, record: ):
log_record = {
"@timestamp": (record, "%Y-%m-%dT%H:%M:%S%z"), # format iso 8601
"remote_addr": getattr(record, "remote_addr", ""),
"scheme": getattr(record, "scheme", ""),
"method": getattr(record, "method", ""),
"host": getattr(record, "host", ""),
"path": getattr(record, "path", ""),
"status": getattr(record, "status", ""),
"response_length": getattr(record, "response_length", ""),
"response_time": getattr(record, "response_time", 0),
}
return (log_record)
class FlaskLogger():
"""Custom log class, set two different loggers, request log and normal log
Args:
name: str, logger name, default is __name__
level: int, log level, default to DEBUG
logfile: str, log file name, default is
logdir: str, log file directory, default to the current directory
access_log: bool, whether it is used to record access logs, default is False
console: bool, whether to output to the console, default is True
json_log: bool, whether to use json format logs, default is True
"""
def __init__(
Self,
name: str = __name__,
level: int = ,
logfile: str = "",
logdir: str = "",
access_log: bool = False,
console: bool = True,
json_log: bool = True,
):
super().__init__(name, level)
= logfile
= logdir
self.access_log = access_log
= console
self.json_log = json_log
self.setup_logpath()
self.setup_handler()
def setup_logpath(self):
"""Set the log file path. If the log directory is not specified when creating the logger, use the current directory """
if not:
Return
p = Path()
if not ():
try:
(parents=True, exist_ok=True)
except Exception as e:
print(f"Failed to create log directory: {e}")
(1)
= p /
def setup_handler(self):
if self.json_log:
formatter = self.set_json_formatter()
else:
formatter = self.set_plain_formatter()
handler_file = self.set_handler_file(formatter)
handler_stdout = self.set_handler_stdout(formatter)
(handler_file)
if :
(handler_stdout)
def set_plain_formatter(self):
fmt = "%(asctime)s | %(levelname)s | %(name)s | %(filename)s:%(lineno)d | %(funcName)s | %(message)s"
datefmt = "%Y-%m-%dT%H:%M:%S%z"
return (fmt, datefmt=datefmt)
def set_json_formatter(self):
"""Set the log in json format"""
if self.access_log:
return AccessLogFormatter()
return JsonFormatter()
def set_handler_stdout(self, formatter: ):
handler = ()
(formatter)
Return handler
def set_handler_file(self, formatter: ):
handler = TimedRotatingFileHandler(
filename=,
When="midnight",
interval=1,
backupCount=7,
encoding="utf-8",
)
(formatter)
Return handlerInstantiation example
access_logger = FlaskLogger("access", logdir="logs", access_log=True, logfile="")
logger = FlaskLogger(logdir="logs")
Record request logs in hook function
With the help of flask's built-in hook functions and global objects, you can record the information of each request.
from flask import g, request, Response
import time
@app.before_request
def start_timer():
# Record the request start time through global object g
g.start_time = ()
@app.after_request
def log_request(response: Response):
"""Login for each request"""
response_length = (
response.content_length if response.content_length is not None else "-"
)
log_message = {
"remote_addr": request.remote_addr,
"method": ,
"scheme": ,
"host": ,
"path": ,
"status": response.status_code,
"response_length": response_length,
"response_time": round(() - g.start_time, 4),
}
access_logger.info("", extra=log_message)
Return responseBasic usage examples
Instantiate Flask objects, disable default logs, define routes, etc.
from flask import Flask
import traceback
app = Flask(__name__)
@(Exception)
def handle_exception(e):
""Global interception exception"""
(f"An exception occurred, {traceback.format_exc()}", exc_info=e)
return "An error occurred", 500
@("/")
def hello():
# Normal request
("Hello World")
return "hello world"
@("/error")
def raise_error():
# Simulate the wrong request and observe whether it is globally captured
raise Exception("Error")
@("/slow")
def slow():
# Simulate slow requests and observe the response time of the request log
(5)
return "slow"
if __name__ == "__main__":
# Disable the default logger
default_logger = ("werkzeug")
default_logger.disabled = True
(host="127.0.0.1", port=5000)Access the test, the logs directory will be generatedandFile, console output example
{"@timestamp": "2025-04-26T00:26:20+0800", "level": "INFO", "name": "__main__", "file": "", "lineno": 162, "func": "hello", "message": "Hello World"}
{"@timestamp": "2025-04-26T00:26:20+0800", "remote_addr": "127.0.0.1", "scheme": "http", "method": "GET", "host": "127.0.0.1:5000", "path": "/", "status": 200, "response_length": 11, "response_time": 0.0003}
{"@timestamp": "2025-04-26T00:26:20+0800", "level": "INFO", "name": "__main__", "file": "", "lineno": 162, "func": "hello", "message": "Hello World"}
{"@timestamp": "2025-04-26T00:26:20+0800", "remote_addr": "127.0.0.1", "scheme": "http", "method": "GET", "host": "127.0.0.1:5000", "path": "/", "status": 200, "response_length": 11, "response_time": 0.0003}
{"@timestamp": "2025-04-26T00:29:47+0800", "remote_addr": "127.0.0.1", "scheme": "http", "method": "GET", "host": "127.0.0.1:5000", "path": "/slow", "status": 200, "response_length": 4, "response_time": 5.0002}
{"@timestamp": "2025-04-26T00:31:02+0800", "level": "ERROR", "name": "__main__", "file": "", "lineno": 129, "func": "handle_exception", "message": "An exception occurred, Traceback (most recent call last):\n File \"/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/lib/python3.11/site-packages/flask/\", line 917, in full_dispatch_request\n rv = self.dispatch_request()\n ^^^^^^^^^^^^^^^^^^^^^^^\n File \"/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/lib/python3.11/site-packages/flask/\", line 902, in dispatch_request\n return self.ensure_sync(self.view_functions[])(**view_args) # type: ignore[no-any-return]\n ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n File \"/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/demo1/\", line 168, in raise_error\n raise Exception(\"Error\")\nException: Error\n"}
{"@timestamp": "2025-04-26T00:31:02+0800", "remote_addr": "127.0.0.1", "scheme": "http", "method": "GET", "host": "127.0.0.1:5000", "path": "/error", "status": 500, "response_length": 17, "response_time": 0.0011}
Complete usage example
from flask import Flask, request, g, Response
import logging
import sys
from import TimedRotatingFileHandler
import json
from pathlib import Path
import traceback
import time
app = Flask(__name__)
class JsonFormatter():
def format(self, record: ):
log_record = {
"@timestamp": (record, "%Y-%m-%dT%H:%M:%S%z"), # format iso 8601
"level": ,
"name": ,
"file": ,
"lineno": ,
"func": ,
"message": (),
}
return (log_record)
class AccessLogFormatter():
def format(self, record: ):
log_record = {
"@timestamp": (record, "%Y-%m-%dT%H:%M:%S%z"), # format iso 8601
"remote_addr": getattr(record, "remote_addr", ""),
"scheme": getattr(record, "scheme", ""),
"method": getattr(record, "method", ""),
"host": getattr(record, "host", ""),
"path": getattr(record, "path", ""),
"status": getattr(record, "status", ""),
"response_length": getattr(record, "response_length", ""),
"response_time": getattr(record, "response_time", 0),
}
return (log_record)
class FlaskLogger():
"""Custom log class, set two different loggers, request log and normal log
Args:
name: str, logger name, default is __name__
level: int, log level, default to DEBUG
logfile: str, log file name, default is
logdir: str, log file directory, default to the current directory
access_log: bool, whether it is used to record access logs, default is False
console: bool, whether to output to the console, default is True
json_log: bool, whether to use json format logs, default is True
"""
def __init__(
Self,
name: str = __name__,
level: int = ,
logfile: str = "",
logdir: str = "",
access_log: bool = False,
console: bool = True,
json_log: bool = True,
):
super().__init__(name, level)
= logfile
= logdir
self.access_log = access_log
= console
self.json_log = json_log
self.setup_logpath()
self.setup_handler()
def setup_logpath(self):
"""Set the log file path. If the log directory is not specified when creating the logger, use the current directory """
if not:
Return
p = Path()
if not ():
try:
(parents=True, exist_ok=True)
except Exception as e:
print(f"Failed to create log directory: {e}")
(1)
= p /
def setup_handler(self):
if self.json_log:
formatter = self.set_json_formatter()
else:
formatter = self.set_plain_formatter()
handler_file = self.set_handler_file(formatter)
handler_stdout = self.set_handler_stdout(formatter)
(handler_file)
if :
(handler_stdout)
def set_plain_formatter(self):
fmt = "%(asctime)s | %(levelname)s | %(name)s | %(filename)s:%(lineno)d | %(funcName)s | %(message)s"
datefmt = "%Y-%m-%dT%H:%M:%S%z"
return (fmt, datefmt=datefmt)
def set_json_formatter(self):
"""Set the log in json format"""
if self.access_log:
return AccessLogFormatter()
return JsonFormatter()
def set_handler_stdout(self, formatter: ):
handler = ()
(formatter)
Return handler
def set_handler_file(self, formatter: ):
handler = TimedRotatingFileHandler(
filename=,
When="midnight",
interval=1,
backupCount=7,
encoding="utf-8",
)
(formatter)
Return handler
access_logger = FlaskLogger("access", logdir="logs", access_log=True, logfile="")
logger = FlaskLogger(logdir="logs")
@(Exception)
def handle_exception(e):
""Global interception exception"""
(f"An exception occurred, {traceback.format_exc()}", exc_info=e)
return "An error occurred", 500
@app.before_request
def start_timer():
# Record the request start time through global object g
g.start_time = ()
@app.after_request
def log_request(response: Response):
"""Login for each request"""
response_length = (
response.content_length if response.content_length is not None else "-"
)
log_message = {
"remote_addr": request.remote_addr,
"method": ,
"scheme": ,
"host": ,
"path": ,
"status": response.status_code,
"response_length": response_length,
"response_time": round(() - g.start_time, 4),
}
access_logger.info("", extra=log_message)
Return response
@("/")
def hello():
# Normal request
("Hello World")
return "hello world"
@("/error")
def raise_error():
# Simulate the wrong request and observe whether it is globally captured
raise Exception("Error")
@("/slow")
def slow():
# Simulate slow requests and observe the response time of the request log
(5)
return "slow"
if __name__ == "__main__":
# Disable the default logger
default_logger = ("werkzeug")
default_logger.disabled = True
(host="127.0.0.1", port=5000)refer to
- Flask - logging
- Python - logging