pkg pprint

Reflection-driven pretty printer for Mojo structs.

• Zero boilerplate: works on any struct without implementing traits
• Configurable: custom indentation, depth limits, item limits
• Type annotations: optional show_types mode
• JSON-style output: booleans as true/false, strings quoted

Why pprint?

Mojo's built-in print() requires types to implement the Writable trait:

@fieldwise_init
struct Person(Copyable, Movable):
    var name: String
    var age: Int

def main():
    var p = Person("Ada", 36)
    print(p)  # ERROR: 'Person' does not implement 'Writable'

With pprint, no trait implementation is required - it uses reflection to inspect and format any struct automatically:

from pprint import pprint

def main():
    var p = Person("Ada", 36)
    pprint(p)  # Works! No Writable needed

Requirements

pixi package manager

Installation

Add pprint to your project's pixi.toml:

[workspace]
channels = ["https://conda.modular.com/max-nightly", "conda-forge"]
preview = ["pixi-build"]

[dependencies]
pprint = { git = "https://github.com/ehsanmok/pprint.git" }

Then run:

pixi install

Quick Start

from pprint import pprint, PrettyPrinter

@fieldwise_init
struct Person(Copyable, Movable):
    var name: String
    var age: Int

def main():
    var p = Person("Ada", 36)
    pprint(p)

Output:

{
  name: "Ada",
  age: 36
}

API Reference

pprint() - Print to stdout

Print any value to stdout with pretty formatting. Like Python's pprint.pprint.

from pprint import pprint, PrettyPrinter

# Default formatting
pprint(my_struct)

# With custom configuration
var pp = PrettyPrinter(indent=4, show_types=True)
pprint(my_struct, pp)

pformat() - Format as String

Return a pretty-formatted string instead of printing. Like Python's pprint.pformat. Useful for logging, string concatenation, or storing formatted output.

from pprint import pformat, PrettyPrinter

# Get formatted string
var s = pformat(my_struct)
print("Debug: " + s)

# With custom configuration
var pp = PrettyPrinter(indent=4)
var s = pformat(my_struct, pp)

PrettyPrinter - Configuration

All formatting options are controlled through PrettyPrinter:

var pp = PrettyPrinter(
    indent=2,         # Spaces per indent level (default: 2)
    max_depth=6,      # Max nesting depth before "..." (default: 6)
    max_items=64,     # Max fields shown before "..." (default: 64)
    show_types=False, # Show type annotations (default: False)
    compact=False,    # Reserved for future use
)

Supported Types

Examples

Scalars

pprint and pformat work directly on scalar values:

from pprint import pformat

print(pformat(42))            # 42
print(pformat(True))          # true
print(pformat(3.14))          # 3.14
print(pformat("hello world")) # "hello world"

Nested Structs

Nested structs are formatted recursively with proper indentation:

from pprint import pprint

@fieldwise_init
struct Address(Copyable, Movable):
    var city: String
    var zip: Int

@fieldwise_init
struct Person(Copyable, Movable):
    var name: String
    var age: Int
    var active: Bool
    var score: Float64
    var address: Address

def main():
    var p = Person("Ada", 36, True, 95.5, Address("London", 12345))
    pprint(p)

Output:

{
  name: "Ada",
  age: 36,
  active: true,
  score: 95.5,
  address: {
    city: "London",
    zip: 12345
  }
}

Type Annotations

Enable show_types=True to see type information after each value:

from pprint import pprint, PrettyPrinter

var pp = PrettyPrinter(show_types=True)
pprint(my_struct, pp)

Output:

{
  name: "Ada" <String>,
  age: 36 <Int>,
  active: true <Bool>,
  score: 95.5 <Float64>,
  address: {
    city: "London" <String>,
    zip: 12345 <Int>
  } <Address>
}

Custom Indentation

Control indent width for different readability needs:

from pprint import pprint, PrettyPrinter

# 4-space indent
pprint(my_struct, PrettyPrinter(indent=4))

# Minimal indent
pprint(my_struct, PrettyPrinter(indent=1))

Depth Limiting

Prevent overly deep output for complex nested structures:

from pprint import pprint, PrettyPrinter

# Truncate at depth 2 (shows "..." for deeper levels)
pprint(deeply_nested, PrettyPrinter(max_depth=2))

Output:

{
  inner: {
    inner: ...
  }
}

Field Limiting

Limit the number of fields shown for structs with many fields:

from pprint import pprint, PrettyPrinter

# Show only first 3 fields, then "..."
pprint(large_struct, PrettyPrinter(max_items=3))

Using pformat for Logging

from pprint import pformat

def log_state[T: AnyType](label: String, value: T):
    print("[LOG] " + label + ": " + pformat(value))

def main():
    var config = AppConfig(...)
    log_state("startup config", config)