Learn how to properly use Python temporary files

1. Preface

Temporary files are usually used to save data that cannot be saved in memory, or to pass to data that must be read from the file. external program. Generally we will generate a unique file name in the /tmp directory, but creating temporary files safely is not that simple and needs to follow many rules. Never try to do this yourself, instead use library functions to do it. Also be careful to clean up temporary files.

The biggest problem caused by temporary files is that the file name can be predicted, allowing malicious users to predict the temporary file name and create soft links to hijack the temporary file.

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##2. Introduction to the tempfile module

The module generally used to create temporary files is tempfile. The commonly used functions of this module library are the following:

tempfile.mktemp # Unsafe, prohibited to use
• tempfile.mkstemp # Randomly create tmp files. The files created by default are in the /tmp directory. Of course, you can also specify (can use)
• tempfile.TemporaryFile # Create files in memory. The files will not be stored on the disk. After closing That is, delete (can be used)
• tempfile.NamedTemporaryFile(delete=True) When delete=True, the effect is the same as above. When it is False, it will be stored on the disk (can be used)

3. Example introduction

The following methods introduce safe and unsafe ways to create temporary files.

3.1 Incorrect example:

Incorrect 1:

import os
import tempfile
 
# This will most certainly put you at risk
tmp = os.path.join(tempfile.gettempdir(), filename)
if not os.path.exists(tmp):
    with open(tmp, "w") file:
        file.write("defaults")

Incorrect 2:

import os
import tempfile
 
open(tempfile.mktemp(), "w")

Incorrect 3:

filename = "{}/{}.tmp".format(tempfile.gettempdir(), os.getpid())
open(filename, "w")

3.2 Correct example

Correct 1:

fd, path = tempfile.mkstemp()
try:
    with os.fdopen(fd, 'w') as tmp:
        # do stuff with temp file
        tmp.write('stuff')
finally:
    os.remove(path)

Correct 2:

# 句柄关闭，文件即删除
with tempfile.TemporaryFile() as tmp:
    # Do stuff with tmp
    tmp.write('stuff')

Correct 3:

tmp = tempfile.NamedTemporaryFile(delete=True)
try:
    # do stuff with temp
    tmp.write('stuff')
finally:
    tmp.close()  # 文件关闭即删除

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