# Temporary file handling¶

AUTHORS:

class sage.misc.temporary_file.atomic_write(target_filename, append=False, mode=438, binary=None, **kwargs)

Bases: object

Write to a given file using a temporary file and then rename it to the target file. This renaming should be atomic on modern operating systems. Therefore, this class can be used to avoid race conditions when a file might be read while it is being written. It also avoids having partially written files due to exceptions or crashes.

This is to be used in a with statement, where a temporary file is created when entering the with and is moved in place of the target file when exiting the with (if no exceptions occurred).

INPUT:

• target_filename – the name of the file to be written. Normally, the contents of this file will be overwritten.

• append – (boolean, default: False) if True and target_filename is an existing file, then copy the current contents of target_filename to the temporary file when entering the with statement. Otherwise, the temporary file is initially empty.

• mode – (default: 0o666) mode bits for the file. The temporary file is created with mode mode & ~umask and the resulting file will also have these permissions (unless the mode bits of the file were changed manually). (Not to be confused with the file opening mode.)

• binary – (boolean, default: True on Python 2, False on Python 3) the underlying file is opened in binary mode. If False then it is opened in text mode and an encoding with which to write the file may be supplied.

• **kwargs – additional keyword arguments passed to the underlying $$io.open$$ call.

EXAMPLES:

sage: from sage.misc.temporary_file import atomic_write
sage: target_file = tmp_filename()
sage: with open(target_file, 'w') as f:
....:     _ = f.write("Old contents")
sage: with atomic_write(target_file) as f:
....:     _ = f.write("New contents")
....:     f.flush()
....:     with open(target_file, 'r') as f2:
'Old contents'
sage: with open(target_file, 'r') as f:
'New contents'


The name of the temporary file can be accessed using f.name. It is not a problem to close and re-open the temporary file:

sage: from sage.misc.temporary_file import atomic_write
sage: target_file = tmp_filename()
sage: with open(target_file, 'w') as f:
....:     _ = f.write("Old contents")
sage: with atomic_write(target_file) as f:
....:     f.close()
....:     with open(f.name, 'w') as f2:
....:         _ = f2.write("Newer contents")
sage: with open(target_file, 'r') as f:


If an exception occurs while writing the file, the target file is not touched:

sage: with atomic_write(target_file) as f:
....:     _ = f.write("Newest contents")
....:     raise RuntimeError
Traceback (most recent call last):
...
RuntimeError
sage: with open(target_file, 'r') as f:


Some examples of using the append option. Note that the file is never opened in “append” mode, it is possible to overwrite existing data:

sage: target_file = tmp_filename()
sage: with atomic_write(target_file, append=True) as f:
....:     _ = f.write("Hello")
sage: with atomic_write(target_file, append=True) as f:
....:     _ = f.write(" World")
sage: with open(target_file, 'r') as f:
'Hello World'
sage: with atomic_write(target_file, append=True) as f:
....:     _ = f.seek(0)
....:     _ = f.write("HELLO")
sage: with open(target_file, 'r') as f:
'HELLO World'


If the target file is a symbolic link, the link is kept and the target of the link is written to:

sage: link_to_target = os.path.join(tmp_dir(), "templink")
sage: with atomic_write(link_to_target) as f:
....:     _ = f.write("Newest contents")
sage: with open(target_file, 'r') as f:


We check the permission bits of the new file. Note that the old permissions do not matter:

sage: os.chmod(target_file, 0o600)
sage: _ = os.umask(0o022)
sage: with atomic_write(target_file) as f:
....:     pass
sage: '{:#o}'.format(os.stat(target_file).st_mode & 0o777)
'0o644'
sage: _ = os.umask(0o077)
sage: with atomic_write(target_file, mode=0o777) as f:
....:     pass
sage: '{:#o}'.format(os.stat(target_file).st_mode & 0o777)
'0o700'


Test writing twice to the same target file. The outermost with “wins”:

sage: with open(target_file, 'w') as f:
....:     _ = f.write('>>> ')
sage: with atomic_write(target_file, append=True) as f,         ....:          atomic_write(target_file, append=True) as g:
....:     _ = f.write("AAA"); f.close()
....:     _ = g.write("BBB"); g.close()
sage: with open(target_file, 'r') as f:
'>>> AAA'


Supplying an encoding means we’re writing the file in “text mode” (in the same sense as $$io.open$$) and so we must write unicode strings:

sage: target_file = tmp_filename()
sage: with atomic_write(target_file, binary=False,
....:                   encoding='utf-8') as f:
....:     _ = f.write(u'Hélas')
sage: import io
sage: with io.open(target_file, encoding='utf-8') as f:
Hélas


Supplying an encoding in binary mode (or other arguments that don’t make sense to $$io.open$$ in binary mode) is an error:

sage: writer = atomic_write(target_file, binary=True,
....:                       encoding='utf-8')
sage: with writer as f:
....:     _ = f.write(u'Hello')
Traceback (most recent call last):
...
ValueError: binary mode doesn't take an encoding argument
sage: os.path.exists(writer.tempname)
False

sage.misc.temporary_file.delete_tmpfiles()

Remove the directory SAGE_TMP.

sage.misc.temporary_file.tmp_dir(name='dir_', ext='')

Create and return a temporary directory in $HOME/.sage/temp/hostname/pid/ The temporary directory is deleted automatically when Sage exits. INPUT: • name – (default: "dir_") A prefix for the directory name. • ext – (default: "") A suffix for the directory name. OUTPUT: The absolute path of the temporary directory created, with a trailing slash (or whatever the path separator is on your OS). EXAMPLES: sage: d = tmp_dir('dir_testing_', '.extension') sage: d # random output '/home/username/.sage/temp/hostname/7961/dir_testing_XgRu4p.extension/' sage: os.chdir(d) sage: f = open('file_inside_d', 'w')  Temporary directories are unaccessible by other users: sage: os.stat(d).st_mode & 0o077 0 sage: f.close()  sage.misc.temporary_file.tmp_filename(name='tmp_', ext='') Create and return a temporary file in $HOME/.sage/temp/hostname/pid/

The temporary file is deleted automatically when Sage exits.

Warning

If you need a particular file extension always use tmp_filename(ext=".foo"), this will ensure that the file does not yet exist. If you were to use tmp_filename()+".foo", then you might overwrite an existing file!

INPUT:

• name – (default: "tmp_") A prefix for the file name.

• ext – (default: "") A suffix for the file name. If you want a filename extension in the usual sense, this should start with a dot.

OUTPUT:

The absolute path of the temporary file created.

EXAMPLES:

sage: fn = tmp_filename('just_for_testing_', '.extension')
sage: fn  # random

sage: os.stat(fn).st_mode & 0o077