Icecream: Never use print() to debug again in Python

0
10




IceCream — Never use print() to debug again

Do you ever use print() or log() to debug your code? Of course you
do. IceCream, or ic for short, makes print debugging a little sweeter.

IceCream is well tested, permissively licensed, and supports
Python 2, Python 3, PyPy2, and PyPy3.

Inspect Variables

Have you ever printed variables or expressions to debug your program? If you’ve
ever typed something like

or the more thorough

print("foo('123')", foo('123'))

then ic() is here to help. With arguments, ic() inspects itself and prints
both its own arguments and the values of those arguments.

from icecream import ic

def foo(i):
    return i + 333

ic(foo(123))

Prints

ic| foo(123): 456

Similarly,

d = {'key': {1: 'one'}}
ic(d['key'][1])

class klass():
    attr = 'yep'
ic(klass.attr)

Prints

ic| d['key'][1]: 'one'
ic| klass.attr: 'yep'

Just give ic() a variable or expression and you’re done. Easy.

Inspect Execution

Have you ever used print() to determine which parts of your program are
executed, and in which order they’re executed? For example, if you’ve ever added
print statements to debug code like

def foo():
    print(0)
    first()

    if expression:
        print(1)
        second()
    else:
        print(2)
        third()

then ic() helps here, too. Without arguments, ic() inspects itself and
prints the calling filename, line number, and parent function.

from icecream import ic

def foo():
    ic()
    first()
    
    if expression:
        ic()
        second()
    else:
        ic()
        third()

Prints

ic| example.py:4 in foo()
ic| example.py:11 in foo()

Just call ic() and you’re done. Simple.

Return Value

ic() returns its argument(s), so ic() can easily be inserted into
pre-existing code.

>>> a = 6
>>> def half(i):
>>>     return i / 2
>>> b = half(ic(a))
ic| a: 6
>>> ic(b)
ic| b: 3

Miscellaneous

ic.format(*args) is like ic() but the output is returned as a string instead
of written to stderr.

>>> from icecream import ic
>>> s = 'sup'
>>> out = ic.format(s)
>>> print(out)
ic| s: 'sup'

Additionally, ic()‘s output can be entirely disabled, and later re-enabled, with
ic.disable() and ic.enable() respectively.

from icecream import ic

ic(1)

ic.disable()
ic(2)

ic.enable()
ic(3)

Prints

ic| 1: 1
ic| 3: 3

ic() continues to return its arguments when disabled, of course; no existing
code with ic() breaks.

Import Tricks

To make ic() available in every file without needing to be imported in
every file, you can install() it. For example, in a root A.py:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from icecream import install
install()

from B import foo
foo()

and then in B.py, which is imported by A.py, just call ic():

# -*- coding: utf-8 -*-

def foo():
    x = 3
    ic(x)

install() adds ic() to the
builtins module,
which is shared amongst all files imported by the interpreter.
Similarly, ic() can later be uninstall()ed, too.

ic() can also be imported in a manner that fails gracefully if
IceCream isn’t installed, like in production environments (i.e. not
development). To that end, this fallback import snippet may prove
useful:

try:
    from icecream import ic
except ImportError:  # Graceful fallback if IceCream isn't installed.
    ic = lambda *a: None if not a else (a[0] if len(a) == 1 else a)  # noqa

Configuration

ic.configureOutput(prefix, outputFunction, argToStringFunction, includeContext) can be used to adopt a custom output prefix (the default is
ic| ), change the output function (default is to write to stderr), customize
how arguments are serialized to strings, and/or include the ic() call’s
context (filename, line number, and parent function) in ic() output with
arguments.

>>> from icecream import ic
>>> ic.configureOutput(prefix='hello -> ')
>>> ic('world')
hello -> 'world'

prefix can optionally be a function, too.

>>> import time
>>> from icecream import ic
>>>  
>>> def unixTimestamp():
>>>     return '%i |> ' % int(time.time())
>>>
>>> ic.configureOutput(prefix=unixTimestamp)
>>> ic('world')
1519185860 |> 'world': 'world'

outputFunction, if provided, is called with ic()‘s output instead of that
output being written to stderr (the default).

>>> import logging
>>> from icecream import ic
>>>
>>> def warn(s):
>>>     logging.warning(s)
>>>
>>> ic.configureOutput(outputFunction=warn)
>>> ic('eep')
WARNING:root:ic| 'eep': 'eep'

argToStringFunction, if provided, is called with argument values to be
serialized to displayable strings. The default is PrettyPrint’s
pprint.pformat(),
but this can be changed to, for example, handle non-standard datatypes in a
custom fashion.

>>> from icecream import ic
>>> 
>>> def toString(obj):
>>>    if isinstance(obj, str):
>>>        return '[!string %r with length %i!]' % (obj, len(obj))
>>>    return repr(obj)
>>> 
>>> ic.configureOutput(argToStringFunction=toString)
>>> ic(7, 'hello')
ic| 7: 7, 'hello': [!string 'hello' with length 5!]

includeContext, if provided and True, adds the ic() call’s filename, line
number, and parent function to ic()‘s output.

>>> from icecream import ic
>>> ic.configureOutput(includeContext=True)
>>> 
>>> def foo():
>>>   ic('str')
>>> foo()
ic| example.py:12 in foo()- 'str': 'str'

includeContext is False by default.

Installation

Installing IceCream with pip is easy.

$ pip install icecream

IceCream in Other Languages

IceCream should be enjoyed with every language.

If you’d like a similar ic() function in your favorite language, please open a
pull request! IceCream’s goal is to sweeten print debugging with a handy-dandy
ic() function in every language.

LEAVE A REPLY

Please enter your comment!
Please enter your name here