File Handling in Python

2.1 Core concepts

• What is a file? A file is a named location on a secondary storage media where data are permanently stored for later access. Programs written in script mode are stored with a .py extension; data output can also be stored permanently in a file. (NCERT §2.1, p. 19)
• Why files are needed: Variables in a program have a lifetime only till the program is under execution. To reuse input data or generated output (e.g., employee information, inventory, sales), data must be stored permanently on secondary storage devices. (NCERT §2.1, p. 19)
• Text files: A text file is a sequence of characters consisting of alphabets, numbers, and other special symbols. Extensions like .txt, .py, .csv are examples. Internally, characters are stored as bytes using ASCII, UNICODE, or another encoding scheme. Each line is terminated by a special End of Line (EOL) character; in Python the default EOL is the newline character \n. (NCERT §2.2.1, p. 20)
• Binary files: Binary files are stored in terms of bytes (0s and 1s) that represent actual content such as images, audio, video, compressed files, and executable files. They are not human-readable; opening one in a text editor yields garbage values. A single bit change can corrupt a binary file. Specific software is required to read or write binary file contents. (NCERT §2.2.2, p. 21)
• Opening a file — open(): Syntax is file_object = open(file_name, access_mode). The function returns a file handle stored in file_object. The file_object has attributes: <file.closed> (True if closed), <file.mode> (access mode), <file.name> (file name). If the file does not exist, open() in write mode creates a new empty file. (NCERT §2.3.1, p. 21–22)
• File access modes (Table 2.1): Key modes — r (read-only, offset at beginning), rb (binary read-only), r+ or +r (read and write, beginning), w (write, overwrites existing content or creates new file), wb+ or +wb (read, write, binary), a (append, offset at end; creates file if absent), a+ or +a (append and read, offset at end). Default mode is read (r); default file type is text. (NCERT §2.3.1, p. 22)
• Closing a file — close(): Syntax: file_object.close(). Frees memory allocated to the file. Python flushes any unwritten/unsaved data to the file before closing. If a file object is re-assigned to another file, the previous file is automatically closed. (NCERT §2.3.2, p. 23)
• with clause: Syntax: with open(file_name, access_mode) as file_object:. The file is closed automatically when control exits the with block, even if an exception occurs. No explicit close() is needed. (NCERT §2.3.3, p. 23)
• Writing — write(): Takes a single string argument and writes it to the text file. Returns the number of characters written. Numeric data must be converted to string using str() before writing. A \n must be added manually to mark end of line. write() actually writes data onto a buffer; the buffer is moved to permanent storage only when close() is called (or flush() is explicitly invoked). (NCERT §2.4.1, p. 24)
• Writing — writelines(): Writes a sequence of strings (from an iterable such as a list or tuple). Does not return the number of characters written. Does not add newline characters automatically. (NCERT §2.4.2, p. 24–25)
• Reading — read([n]): Reads and returns n bytes from the file. If no argument or a negative number is given, the entire file content is read and returned as a single string. (NCERT §2.5.1, p. 25)
• Reading — readline([n]): Reads one complete line (up to and including \n) from the file. If n is specified, reads at most n bytes from that line. Returns an empty string at EOF. Useful for iterating line by line with a loop. (NCERT §2.5.2, p. 26)
• Reading — readlines(): Reads all lines from the file and returns them as a list of strings, each ending with \n. split() separates words within a line; splitlines() returns each line as a list element without the newline character. (NCERT §2.5.3, p. 26–27)
• tell(): Returns an integer giving the current byte position of the file object from the beginning of the file. Syntax: file_object.tell(). (NCERT §2.6.1, p. 28)
• seek(): Positions the file object at a specified location. Syntax: file_object.seek(offset [, reference_point]). reference_point values: 0 = beginning (default), 1 = current position, 2 = end of file. offset is the number of bytes to move. (NCERT §2.6.2, p. 28)
• Creating and traversing a text file (§2.7): Combining open() in w+ mode, write(), seek(0), and read() allows a single program to both write and read data. Using readline() in a while loop traverses the file line by line until an empty string (EOF) is returned. (NCERT §2.7, p. 29–31)
• Pickle module — serialization: Python's pickle module serializes (pickles) any Python object (list, tuple, dictionary, etc.) into a byte stream that can be stored in a binary file or sent over a network. The reverse process — converting the byte stream back to a Python object — is called de-serialization or unpickling. The pickle module must be imported explicitly. (NCERT §2.8, p. 32)
• dump(): Syntax: pickle.dump(data_object, file_object). Converts (pickles) a Python object and writes it to a binary file opened in wb or ab mode. (NCERT §2.8.1, p. 32)
• load(): Syntax: store_object = pickle.load(file_object). Reads (unpickles) a Python object from a binary file opened in rb mode. At end-of-file, raises EOFError, which can be caught with a try…except block. (NCERT §2.8.2, p. 33)

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Table 2.1 — File Open Modes (p. 22): A tabular summary of all access modes (r, rb, r+, w, wb+, a, a+) with their descriptions and initial file offset positions. Students must memorize which modes start at the beginning vs. the end of the file, and which modes overwrite vs. append.
• Figure 2.1 — Contents of myfile.txt (p. 25): Shows the output of writelines() in Notepad — three lines written without explicit \n separators display correctly only because \n was embedded in each string. Reinforces that writelines() does not auto-insert newlines.
• Program 2-2 — seek() and tell() demonstration (p. 28–29): Illustrates the file object moving from byte 33 (after read()) back to 0 (after seek(0)) and then to byte 10 (after seek(10)). Key to understanding random access.
• Program 2-8 — Binary file with pickle (p. 34–35): Shows the complete workflow — import pickle → open in ab mode → dump() records → close → open in rb mode → load() records in a try…except EOFError loop. The output shows each employee record loaded as a list.

2.4 Common confusions / NTA trap points

• write() vs writelines(): write() accepts only a single string and returns the character count; writelines() accepts an iterable of strings and returns None. Neither method automatically adds \n — the student must include it. NTA often gives code snippets and asks what the output or return value will be.
• Default file mode and type: If no access mode is specified in open(), Python opens the file in read (r) text mode. Many MCQs test whether students know that w truncates existing content while a preserves it.
• read() with no argument vs readline(): read() with no argument reads the entire file as one string; readline() reads exactly one line including the \n. Confusing the two leads to wrong answers in code-trace questions.
• seek() reference point values: 0 = beginning, 1 = current position, 2 = end. NTA likes distractor options that swap these values or claim the default reference point is 1 instead of 0.
• Pickle file modes: dump() requires the file to be opened in binary write (wb) or binary append (ab) mode; load() requires binary read (rb) mode. Opening a pickle file in text mode (w or r) is a common trap.
• with clause auto-close (NCERT §2.3.3, p. 23). Students sometimes think the file remains open after the with block. The file is automatically closed when the block exits.
• w mode truncates! (NCERT Table 2.1, p. 22). Opening an existing file in w deletes its contents. Use a to preserve and append. NTA exploits this.
• open() without mode defaults to r text (NCERT §2.3.1, p. 22). NTA distractor: claims default is w.
• EOF in readline() returns empty string, not None (NCERT §2.5.2, p. 26). Use while line := readline(): style or test for ''.
• pickle.dump() writes ONE object per call (NCERT §2.8.1, p. 32). To store multiple, call dump in a loop.
• tell() returns BYTES not lines (NCERT §2.6.1, p. 28). It is a byte offset.
• seek() reference_point in text mode (NCERT §2.6.2, p. 28). For text files, NCERT recommends only seek(offset, 0) — moving by negative offsets in text mode can be problematic.