How to Read Binary File Bytes and Inspect the Header in Python

Read the first bytes of a binary file with pathlib and display them as a hex dump plus an ASCII view to inspect file headers.

Easy Python 3.9+ Aug 9, 2026 Files & data 12 views 0 copies

binary file-io hex pathlib

Python code

23 lines

Python 3.9+

import pathlib

def inspect_binary_header(filepath: str, num_bytes: int = 16) -> None:
    """Read the first bytes of a binary file and display them as hex and ASCII."""
    path = pathlib.Path(filepath)
    data = path.read_bytes()[:num_bytes]
    
    hex_str = ' '.join(f"{byte:02x}" for byte in data)
    ascii_str = ''.join(chr(byte) if 32 <= byte <= 126 else '.' for byte in data)
    
    print(f"File: {path.name}")
    print(f"Total size: {path.stat().st_size} bytes")
    print(f"First {len(data)} bytes:")
    print(f"  Hex:   {hex_str}")
    print(f"  ASCII: {ascii_str}")

if __name__ == "__main__":
    # Create a sample binary file for demonstration
    sample_path = pathlib.Path("sample.bin")
    sample_path.write_bytes(bytes([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A, 0x00, 0x01, 0x02, 0xFF]))
    
    inspect_binary_header(sample_path)
    sample_path.unlink()  # Clean up the sample file

Output

stdout

File: sample.bin
Total size: 12 bytes
First 12 bytes:
  Hex:   89 50 4e 47 0d 0a 1a 0a 00 01 02 ff
  ASCII: .PNG.......

How it works

pathlib.Path.read_bytes() returns the entire file as a bytes object in one call, so we slice it with [:num_bytes] to get just the header. The hex string is built with a generator expression that formats each byte as a zero-padded two-digit hex value. The ASCII view uses chr() only for printable bytes (32–126) and a period for anything else, making non-text bytes visible. path.stat().st_size gives the total file size without reading the whole file again. This pattern is a lightweight way to sniff file types or validate formats before parsing the full content.

Common mistakes

Using `open(filepath, 'rb')` and `.read(16)` without closing the file — `Path.read_bytes()` handles closing automatically.
Forgetting that byte values are integers 0–255, not characters, when formatting hex.
Slicing before checking whether the file is shorter than the requested bytes, which can yield fewer bytes than expected (handled here by using `len(data)`).

Variations

Use `open(filepath, 'rb') as f: data = f.read(16)` for streaming, which reads only the first 16 bytes without loading the entire file.
Use the `binascii.hexlify` function for a compact hex string: `binascii.hexlify(data).decode()`.

Real-world use cases

Verify that a downloaded file matches an expected magic number (e.g., PNG header) before processing it as an image.
Automatically detect file type in a batch upload system by reading the first few bytes instead of trusting the filename extension.
Troubleshoot corrupt or mislabeled files in ETL pipelines by inspecting the raw header bytes to see the actual format.

How to Read Binary File Bytes and Inspect the Header in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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