Python file like objects

File-Like Objects in Python

Python supports file like objects, that don’t write to the disk but stay in the memory. You can create file like objects with StringIO. From Python version > 3 this is part of the io module. These files live only inside the computer memory, not on the disk. Python can read files from the disk, but this article focuses on files in memory.

StringIO

To start using file-like objects, first import the io module. Then create a new file with io.StringIO() where the parameter is the file contents.

>>> import io >>> >>> myFile = io.StringIO() 

>>> myFile = io.StringIO("Data into the file") >>> myFile.read() 'Data into the file' 

>>> myFile.seek(0) 0 >>> myFile.read() 'Data into the file' >>> 

>>> import io >>> myFile = io.StringIO("Feeling good") >>> data = myFile.read() >>> print(data) Feeling good >>> 

Write file

You can write data into the memory file too, by using the .write() method. This method is part of the object and as parameter takes a string (there’s also regular write file) The .write() method lets you write any data into the file. The usual escape character work \n for a new line.

>>> myFile = io.StringIO("") >>> myFile.write("Write a line into the file\n") >>> myFile.write("Second line.\n") 

>>> data = myFile.getvalue() >>> data 'Write a line into the file\nSecond line.\n' 

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What is exactly a file-like object in Python?

File-like objects are mainly StringIO objects, connected sockets and, well, actual file objects.

If everything goes well, urllib.urlopen() returns a file-like object supporting the necessary methods.

Similar question

simplejson has the calls loads and dumps that consumes and produce strings instead of file like objects.

This link has an example in the context of StringIO and simplejson for both file-like and string objects.

This is the API for all file-like objects in Python (as of 3.10.5).

# All file-like objects inherit the IOBase interface: # Documented at https://docs.python.org/3/library/io.html#io.IOBase . close() -> None closed() -> bool # Implemented as @property `closed` fileno() -> int flush() -> None isatty() -> bool readable() -> bool readline(size: int = -1) -> Union[str, bytes] readlines(hint: Union[int, None] = None) -> list seek(pos: int, whence: int = io.SEEK_SET) -> int # SEEK_SET is 0 seekable() -> bool tell() -> int truncate(pos: int = None) -> int # The parameter is named "size" in class FileIO writable() -> bool writelines(lines: list) -> None __del__() -> None # Documented at https://docs.python.org/3/library/io.html#class-hierarchy . __enter__() __exit__(*args) -> None: __iter__() __next__() -> Union[str, bytes] # Documented in paragraph at https://docs.python.org/3/library/io.html#io.IOBase . # Note that while the documentation claims that the method signatures # of `read` and `write` vary, all file-like objects included in the Python # Standard Library have the following exact method signatures for `read` and `write`: read(size: int = -1) -> Union[str, bytes] write(b: Union[str, bytes]) -> int # The parameter is named "s" in TextIOBase 

Specific file-like objects may implement more than this, but this is the subset of methods that are common to ALL file-like objects.

The IO Class Hierarchy section in the IO documentation contains a table listing the built-in and stub methods for the different types of file-like objects.

Basically, there is a hierarchy of abstract base classes:

• IOBase , which promises little
  • RawIOBase , which provides unbuffered binary IO
  • BufferedIOBase , which provides buffered binary IO
  • TextIOBase , which provides buffered string IO

  To implement a file-like object, you would subclass one of the three descendants of IOBase , but not IOBase itself. See this answer for attempting to determine which of these a given file-like object is.

  Each of these classes provides various stub methods and mixins:

  Class	Stub Methods	Mixins
  IOBase	fileno , seek , truncate	close , closed , __enter__ , __exit__ , flush , isatty , __iter__ , __next__ , readable , readline , readlines , seekable , tell , writable , writelines
  RawIOBase	readinto , write	read , readall
  BufferedIOBase	detach , read , read1 , write	readinto , readinto1
  TextIOBase	detach , read , readline , write	encoding , errors , newlines

  The documentation for these methods can be found in the documentation for the classes, linked above.

  

  Phoenix 1373

  In Python, a file object is an object exposing an API having methods for performing operations typically done on files, such as read() or write() .

  In the question’s example: simplejson.load(fp, . ) , the object passed as fp is only required to have a read() method, callable in the same way as a read() on a file (i.e. accepting an optional parameter size and returning either a str or a bytes object).

  This does not need to be a real file, though, as long as it has a read() method.

  A file-like object is just a synonym for file-object. See Python Glossary.

  

  Adrian W 4213

  file object

  An object exposing a file-oriented API (with methods such as read() or write()) to an underlying resource. Depending on the way it was created, a file object can mediate access to a real on-disk file or to another type of storage or communication device (for example standard input/output, in-memory buffers, sockets, pipes, etc.). File objects are also called file-like objects or streams.

  There are actually three categories of file objects: raw binary files, buffered binary files and text files. Their interfaces are defined in the io module. The canonical way to create a file object is by using the open() function.

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