Basic functionality used in the fastai library
from fastcore.test import *
from nbdev.showdoc import *
from fastcore.nb_imports import *



ifnone(a, b)

b if a is None else a

Since b if a is None else a is such a common pattern, we wrap it in a function. However, be careful, because python will evaluate both a and b when calling ifnone (which it doesn't do if using the if version directly).

test_eq(ifnone(None,1), 1)
test_eq(ifnone(2   ,1), 2)


maybe_attr(o, attr)


Return the attribute attr for object o. If the attribute doesn't exist, then return the object o instead.

class myobj: myattr='foo'

test_eq(maybe_attr(myobj, 'myattr'), 'foo')
test_eq(maybe_attr(myobj, 'another_attr'), myobj)



Lookup a user-supplied list of attributes (flds) of an object and generate a string with the name of each attribute and its corresponding value. The format of this string is key=value, where key is the name of the attribute, and value is the value of the attribute. For each value, attempt to use the __name__ attribute, otherwise fall back to using the value's __repr__ when constructing the string.

class SomeClass:
class AnotherClass:
    __repr__=basic_repr(['c', 'd'])
sc = SomeClass()    
ac = AnotherClass()

test_eq(repr(sc), "SomeClass(a=1, b='foo')")
test_eq(repr(ac), "AnotherClass(c=SomeClass(a=1, b='foo'), d='bar')")



True if x supports __array__ or iloc

(True, False)


listify(o=None, *rest, use_list=False, match=None)

Convert o to a list

Conversion is designed to "do what you mean", e.g:

test_eq(listify('hi'), ['hi'])
test_eq(listify(array(1)), [array(1)])
test_eq(listify(1), [1])
test_eq(listify([1,2]), [1,2])
test_eq(listify(range(3)), [0,1,2])
test_eq(listify(None), [])
test_eq(listify(1,2), [1,2])
arr = np.arange(9).reshape(3,3)
[array([[0, 1, 2],
        [3, 4, 5],
        [6, 7, 8]])]
[array([1, 2])]

Generators are turned into lists too:

gen = (o for o in range(3))
test_eq(listify(gen), [0,1,2])

Use match to provide a length to match:

test_eq(listify(1,match=3), [1,1,1])

If match is a sequence, it's length is used:

test_eq(listify(1,match=range(3)), [1,1,1])

If the listified item is not of length 1, it must be the same length as match:

test_eq(listify([1,1,1],match=3), [1,1,1])
test_fail(lambda: listify([1,1],match=3))


tuplify(o, use_list=False, match=None)

Make o a tuple



true(*args, **kwargs)

Predicate: always True

[(o,true(o)) for o in
[(array(0), False),
 (array(1), True),
 (array([0]), True),
 (array([0, 1]), True),
 (1, True),
 (0, False),
 ('', False),
 (None, False)]

class NullType[source]


An object that is False and can be called, chained, and indexed




Convert None to null



get_class(nm, *fld_names, sup=None, doc=None, funcs=None, **flds)

Dynamically create a class, optionally inheriting from sup, containing fld_names

_t = get_class('_t', 'a', b=2)
t = _t()
test_eq(t.a, None)
test_eq(t.b, 2)
t = _t(1, b=3)
test_eq(t.a, 1)
test_eq(t.b, 3)
t = _t(1, 3)
test_eq(t.a, 1)
test_eq(t.b, 3)
test_eq(repr(t), '_t(a=1, b=3)')
test_eq(t, pickle.loads(pickle.dumps(t)))

Most often you'll want to call mk_class, since it adds the class to your module. See mk_class for more details and examples of use (which also apply to get_class).


mk_class(nm, *fld_names, sup=None, doc=None, funcs=None, mod=None, **flds)

Create a class using get_class and add to the caller's module

Any kwargs will be added as class attributes, and sup is an optional (tuple of) base classes.

mk_class('_t', a=1, sup=dict)
t = _t()
test_eq(t.a, 1)

A __init__ is provided that sets attrs for any kwargs, and for any args (matching by position to fields), along with a __repr__ which prints all attrs. The docstring is set to doc. You can pass funcs which will be added as attrs with the function names.

def foo(self): return 1
mk_class('_t', 'a', sup=dict, doc='test doc', funcs=foo)

t = _t(3, b=2)
test_eq(t.a, 3)
test_eq(t.b, 2)
test_eq(, 1)
test_eq(t.__doc__, 'test doc')


wrap_class(nm, *fld_names, sup=None, doc=None, funcs=None, **flds)

Decorator: makes function a method of a new class nm passing parameters to mk_class

@wrap_class('_t', a=2)
def bar(self,x): return x+1

t = _t()
test_eq(t.a, 2)
test_eq(, 4)

class ignore_exceptions[source]


Context manager to ignore exceptions

with ignore_exceptions(): 
    # Exception will be ignored
    raise Exception


exec_local(code, var_name)

Call exec on code and return the var `var_name

test_eq(exec_local("a=1", "a"), 1)


risinstance(types, obj=None)

Curried isinstance but with args reversed

assert risinstance(int, 1)
assert not risinstance(str, 0)
assert risinstance(int)(1)

types can also be strings:

assert risinstance(('str','int'), 'a')
assert risinstance('str', 'a')
assert not risinstance('int', 'a')


These are used when you need a pass-through function.


noop(x=None, *args, **kwargs)

Do nothing



noops(x=None, *args, **kwargs)

Do nothing (method)

class _t: foo=noops

Infinite Lists

These lists are useful for things like padding an array or adding index column(s) to arrays.

class Inf[source]


Infinite lists

Inf defines the following properties:

  • count: itertools.count()
  • zeros: itertools.cycle([0])
  • ones : itertools.cycle([1])
  • nones: itertools.cycle([None])
test_eq([o for i,o in zip(range(5), Inf.count)],
        [0, 1, 2, 3, 4])

test_eq([o for i,o in zip(range(5), Inf.zeros)],

test_eq([o for i,o in zip(range(5), Inf.ones)],

test_eq([o for i,o in zip(range(5), Inf.nones)],

Operator Functions


in_(a, b=<object object at 0x7f22882f7160>)

Same as operator.in_, or returns partial if 1 arg

assert in_('c', ('b', 'c', 'a'))
assert in_(4, [2,3,4,5])
assert in_('t', 'fastai')
test_fail(in_('h', 'fastai'))

# use in_ as a partial
assert in_('fastai')('t')
assert in_([2,3,4,5])(4)

In addition to in_, the following functions are provided matching the behavior of the equivalent versions in operator: lt gt le ge eq ne add sub mul truediv is_ is_not.

(True, False, True, False)

Similarly to _in, they also have additional functionality: if you only pass one param, they return a partial function that passes that param as the second positional parameter.

(True, False, True, False)


true(*args, **kwargs)

Predicate: always True

assert true(1,2,3)
assert true(False)
assert true(None)
assert true([])



Raises exception e (by default StopException)


gen(func, seq, cond=true)

Like (func(o) for o in seq if cond(func(o))) but handles StopIteration

test_eq(gen(noop, Inf.count, lt(5)),
test_eq(gen(operator.neg, Inf.count, gt(-5)),
test_eq(gen(lambda o:o if o<5 else stop(), Inf.count),


chunked(it, chunk_sz=None, drop_last=False, n_chunks=None)

Return batches from iterator it of size chunk_sz (or return n_chunks total)

Note that you must pass either chunk_sz, or n_chunks, but not both.

t = list(range(10))
test_eq(chunked(t,3),      [[0,1,2], [3,4,5], [6,7,8], [9]])
test_eq(chunked(t,3,True), [[0,1,2], [3,4,5], [6,7,8],    ])

t = map(lambda o:stop() if o==6 else o, Inf.count)
test_eq(chunked(t,3), [[0, 1, 2], [3, 4, 5]])
t = map(lambda o:stop() if o==7 else o, Inf.count)
test_eq(chunked(t,3), [[0, 1, 2], [3, 4, 5], [6]])

t = np.arange(10)
test_eq(chunked(t,3),      [[0,1,2], [3,4,5], [6,7,8], [9]])
test_eq(chunked(t,3,True), [[0,1,2], [3,4,5], [6,7,8],    ])


otherwise(x, tst, y)

y if tst(x) else x

test_eq(otherwise(2+1, gt(3), 4), 3)
test_eq(otherwise(2+1, gt(2), 4), 4)

Attribute Helpers

These functions reduce boilerplate when setting or manipulating attributes or properties of objects.


custom_dir(c, add)

Implement custom __dir__, adding add to cls

custom_dir allows you extract the __dict__ property of a class and appends the list add to it.

class _T: 
    def f(): pass

s = custom_dir(_T(), add=['foo', 'bar'])
assert {'foo', 'bar', 'f'}.issubset(s)

class AttrDict[source]

AttrDict() :: dict

dict subclass that also provides access to keys as attrs

d = AttrDict(a=1,b="two")
test_eq(d.a, 1)
test_eq(d['b'], 'two')
test_eq(d.get('c','nope'), 'nope')
d.b = 2
test_eq(d.b, 2)
test_eq(d['b'], 2)
d['b'] = 3
test_eq(d['b'], 3)
test_eq(d.b, 3)
assert 'a' in dir(d)



Same as typing.get_type_hints but returns {} if not allowed type

def type_hints(f):
    "Same as `typing.get_type_hints` but returns `{}` if not allowed type"
    return typing.get_type_hints(f) if isinstance(f, typing._allowed_types) else {}

Below is a list of allowed types for type hints in python:


For example, type func is allowed so type_hints returns the same value as typing.get_hints:

def f(a:int)->bool: ... # a function with type hints (allowed)
exp = {'a':int,'return':bool}
test_eq(type_hints(f), typing.get_type_hints(f))
test_eq(type_hints(f), exp)

However, class is not an allowed type, so type_hints returns {}:

class _T:
    def __init__(self, a:int=0)->bool: ...
assert not type_hints(_T)



Annotations for o, or type(o)

This supports a wider range of situations than type_hints, by checking type() and __init__ for annotations too:

for o in _T,_T(),_T.__init__,f: test_eq(annotations(o), exp)
assert not annotations(int)
assert not annotations(print)



Get the return annotation of func

def f(x) -> float: return x
test_eq(anno_ret(f), float)

def f(x) -> typing.Tuple[float,float]: return x
test_eq(anno_ret(f), typing.Tuple[float,float])

If your return annotation is None, anno_ret will return NoneType (and not None):

def f(x) -> None: return x

test_eq(anno_ret(f), NoneType)
assert anno_ret(f) is not None # returns NoneType instead of None

If your function does not have a return type, or if you pass in None instead of a function, then anno_ret returns None:

def f(x): return x

test_eq(anno_ret(f), None)
test_eq(anno_ret(None), None) # instead of passing in a func, pass in None


argnames(f, frame=False)

Names of arguments to function or frame f

test_eq(argnames(f), ['x'])



Decorator which uses any parameter annotations as preprocessing functions

def _f(a, b:Path, c:str='', d=0): return (a,b,c,d)

test_eq(_f(1, '.', 3), (1,Path('.'),'3',0))
test_eq(_f(1, '.'), (1,Path('.'),'',0))

def _g(a:int=0)->str: return a

test_eq(_g(4.0), '4')
test_eq(_g(4.4), '4')
test_eq(_g(2), '2')


store_attr(names=None, but='', cast=False, store_args=None, **attrs)

Store params named in comma-separated names from calling context into attrs in self

In it's most basic form, you can use store_attr to shorten code like this:

class T:
    def __init__(self, a,b,c): self.a,self.b,self.c = a,b,c this:

class T:
    def __init__(self, a,b,c): store_attr('a,b,c', self)

This class behaves as if we'd used the first form:

t = T(1,c=2,b=3)
assert t.a==1 and t.b==3 and t.c==2

In addition, it stores the attrs as a dict in __stored_args__, which you can use for display, logging, and so forth.

test_eq(t.__stored_args__, {'a':1, 'b':3, 'c':2})

Since you normally want to use the first argument (often called self) for storing attributes, it's optional:

class T:
    def __init__(self, a,b,c:str): store_attr('a,b,c')

t = T(1,c=2,b=3)
assert t.a==1 and t.b==3 and t.c==2

With cast=True any parameter annotations will be used as preprocessing functions for the corresponding arguments:

class T:
    def __init__(self, a:listify, b, c:str): store_attr('a,b,c', cast=True)

t = T(1,c=2,b=3)
assert t.a==[1] and t.b==3 and t.c=='2'

You can inherit from a class using store_attr, and just call it again to add in any new attributes added in the derived class:

class T2(T):
    def __init__(self, d, **kwargs):

t = T2(d=1,a=2,b=3,c=4)
assert t.a==2 and t.b==3 and t.c==4 and t.d==1

You can skip passing a list of attrs to store. In this case, all arguments passed to the method are stored:

class T:
    def __init__(self, a,b,c): store_attr()

t = T(1,c=2,b=3)
assert t.a==1 and t.b==3 and t.c==2
class T4(T):
    def __init__(self, d, **kwargs):

t = T4(4, a=1,c=2,b=3)
assert t.a==1 and t.b==3 and t.c==2 and t.d==4
class T4:
    def __init__(self, *, a: int, b: float = 1):
t = T4(a=3)
assert t.a==3 and t.b==1
t = T4(a=3, b=2)
assert t.a==3 and t.b==2

You can skip some attrs by passing but:

class T:
    def __init__(self, a,b,c): store_attr(but='a')

t = T(1,c=2,b=3)
assert t.b==3 and t.c==2
assert not hasattr(t,'a')

You can also pass keywords to store_attr, which is identical to setting the attrs directly, but also stores them in __stored_args__.

class T:
    def __init__(self): store_attr(a=1)

t = T()
assert t.a==1

You can also use store_attr inside functions.

def create_T(a, b):
    t = SimpleNamespace()
    return t

t = create_T(a=1, b=2)
assert t.a==1 and t.b==2


attrdict(o, *ks, default=None)

Dict from each k in ks to getattr(o,k)

class T:
    def __init__(self, a,b,c): store_attr()

t = T(1,c=2,b=3)
test_eq(attrdict(t,'b','c'), {'b':3, 'c':2})



Change attrs in cls with names in ps to properties

class T:
    def a(self): return 1
    def b(self): return 2



camel2words(s, space=' ')

Convert CamelCase to 'spaced words'

test_eq(camel2words('ClassAreCamel'), 'Class Are Camel')



Convert CamelCase to snake_case

test_eq(camel2snake('ClassAreCamel'), 'class_are_camel')
test_eq(camel2snake('Already_Snake'), 'already__snake')



Convert snake_case to CamelCase

test_eq(snake2camel('a_b_cc'), 'ABCc')



Return the snake-cased name of the class; strip ending cls_name if it exists.

class Parent:
    def name(self): return class2attr(self, 'Parent')

class ChildOfParent(Parent): pass
class ParentChildOf(Parent): pass

p = Parent()
cp = ChildOfParent()
cp2 = ParentChildOf()

test_eq(, 'parent')
test_eq(, 'child_of')
test_eq(, 'parent_child_of')


getattrs(o, *attrs, default=None)

List of all attrs in o

from fractions import Fraction
getattrs(Fraction(1,2), 'numerator', 'denominator')
[1, 2]


hasattrs(o, attrs)

Test whether o contains all attrs

assert hasattrs(1,('imag','real'))
assert not hasattrs(1,('imag','foo'))


setattrs(dest, flds, src)

d = dict(a=1,bb="2",ignore=3)
o = SimpleNamespace()
setattrs(o, "a,bb", d)
test_eq(o.a, 1)
test_eq(, "2")
d = SimpleNamespace(a=1,bb="2",ignore=3)
o = SimpleNamespace()
setattrs(o, "a,bb", d)
test_eq(o.a, 1)
test_eq(, "2")


try_attrs(obj, *attrs)

Return first attr that exists in obj

test_eq(try_attrs(1, 'real'), 1)
test_eq(try_attrs(1, 'foobar', 'real'), 1)

Attribute Delegation

class GetAttrBase[source]


Basic delegation of __getattr__ and __dir__

class GetAttr[source]


Inherit from this to have all attr accesses in self._xtra passed down to self.default

Inherit from GetAttr to have attr access passed down to an instance attribute. This makes it easy to create composites that don't require callers to know about their components. For a more detailed discussion of how this works as well as relevant context, we suggest reading the delegated composition section of this blog article.

You can customise the behaviour of GetAttr in subclasses via;

  • _default
    • By default, this is set to 'default', so attr access is passed down to self.default
    • _default can be set to the name of any instance attribute that does not start with dunder __
  • _xtra
    • By default, this is None, so all attr access is passed down
    • You can limit which attrs get passed down by setting _xtra to a list of attribute names

To illuminate the utility of GetAttr, suppose we have the following two classes, _WebPage which is a superclass of _ProductPage, which we wish to compose like so:

class _WebPage:
    def __init__(self, title, author="Jeremy"):
        self.title, = title,author

class _ProductPage:
    def __init__(self, page, price):,self.price = page,price
page = _WebPage('Soap', author="Sylvain")
p = _ProductPage(page, 15.0)

How do we make it so we can just write, instead of to access the author attribute? We can use GetAttr, of course! First, we subclass GetAttr when defining _ProductPage. Next, we set self.default to the object whose attributes we want to be able to access directly, which in this case is the page argument passed on initialization:

class _ProductPage(GetAttr):
    def __init__(self, page, price): self.default,self.price = page,price #self.default allows you to access page directly.

p = _ProductPage(page, 15.0)

Now, we can access the author attribute directly from the instance:

test_eq(, 'Sylvain')

If you wish to store the object you are composing in an attribute other than self.default, you can set the class attribute _data as shown below. This is useful in the case where you might have a name collision with self.default:

class _C(GetAttr):
    _default = '_data' # use different component name; `self._data` rather than `self.default`
    def __init__(self,a): self._data = a
    def foo(self): noop

t = _C('Hi')
test_eq(t._data, 'Hi') 
test_fail(lambda: t.default) # we no longer have self.default
test_eq(t.lower(), 'hi')
test_eq(t.upper(), 'HI')
assert 'lower' in dir(t)
assert 'upper' in dir(t)

By default, all attributes and methods of the object you are composing are retained. In the below example, we compose a str object with the class _C. This allows us to directly call string methods on instances of class _C, such as str.lower() or str.upper():

class _C(GetAttr):
    # allow all attributes and methods to get passed to `self.default` (by leaving _xtra=None)
    def __init__(self,a): self.default = a
    def foo(self): noop

t = _C('Hi')
test_eq(t.lower(), 'hi')
test_eq(t.upper(), 'HI')
assert 'lower' in dir(t)
assert 'upper' in dir(t)

However, you can choose which attributes or methods to retain by defining a class attribute _xtra, which is a list of allowed attribute and method names to delegate. In the below example, we only delegate the lower method from the composed str object when defining class _C:

class _C(GetAttr):
    _xtra = ['lower'] # specify which attributes get passed to `self.default`
    def __init__(self,a): self.default = a
    def foo(self): noop

t = _C('Hi')
test_eq(t.default, 'Hi')
test_eq(t.lower(), 'hi')
test_fail(lambda: t.upper()) # upper wasn't in _xtra, so it isn't available to be called
assert 'lower' in dir(t)
assert 'upper' not in dir(t)

You must be careful to properly set an instance attribute in __init__ that corresponds to the class attribute _default. The below example sets the class attribute _default to data, but erroneously fails to define (and instead defines self.default).

Failing to properly set instance attributes leads to errors when you try to access methods directly:

class _C(GetAttr):
    _default = 'data' # use a bad component name; i.e. does not exist
    def __init__(self,a): self.default = a
    def foo(self): noop
# TODO: should we raise an error when we create a new instance ...
t = _C('Hi')
test_eq(t.default, 'Hi')
# ... or is it enough for all GetAttr features to raise errors
test_fail(lambda: t.lower())
test_fail(lambda: t.upper())
test_fail(lambda: dir(t))


delegate_attr(k, to)

Use in __getattr__ to delegate to attr to without inheriting from GetAttr

delegate_attr is a functional way to delegate attributes, and is an alternative to GetAttr. We recommend reading the documentation of GetAttr for more details around delegation.

You can use achieve delegation when you define __getattr__ by using delegate_attr:

class _C:
    def __init__(self, o): self.o = o # self.o corresponds to the `to` argument in delegate_attr.
    def __getattr__(self, k): return delegate_attr(self, k, to='o')

t = _C('HELLO') # delegates to a string
test_eq(t.lower(), 'hello')

t = _C(np.array([5,4,3])) # delegates to a numpy array
test_eq(t.sum(), 12)

t = _C(pd.DataFrame({'a': [1,2], 'b': [3,4]})) # delegates to a pandas.DataFrame
test_eq(t.b.max(), 4)

Extensible Types

ShowPrint is a base class that defines a show method, which is used primarily for callbacks in fastai that expect this method to be defined.

Int, Float, and Str extend int, float and str respectively by adding an additional show method by inheriting from ShowPrint.

The code for Int is shown below:



Collection functions

Functions that manipulate popular python collections.



Concatenate all collections in colls

concat([(o for o in range(2)),[2,3,4]])
[0, 1, 2, 3, 4]


strcat(its, sep:str='')

Concatenate stringified items its

test_eq(strcat(['a',2]), 'a2')
test_eq(strcat(['a',2], ';'), 'a;2')



If x is a tuple with one thing, extract it

test_eq(detuplify([1,2]), [1,2])
test_eq(detuplify(np.array([[1,2]])), np.array([[1,2]]))


replicate(item, match)

Create tuple of item copied len(match) times

t = [1,1]
test_eq(replicate([1,2], t),([1,2],[1,2]))
test_eq(replicate(1, t),(1,1))



Turn any list like-object into a set.




Merge all dictionaries in ds

test_eq(merge(), {})
test_eq(merge(dict(a=1,b=2)), dict(a=1,b=2))
test_eq(merge(dict(a=1,b=2), dict(b=3,c=4), None), dict(a=1, b=3, c=4))


range_of(a, b=None, step=None)

All indices of collection a, if a is a collection, otherwise range

test_eq(range_of([1,1,1,1]), [0,1,2,3])


groupby(x, key, val=noop)

Like itertools.groupby but doesn't need to be sorted, and isn't lazy, plus some extensions

test_eq(groupby('aa ab bb'.split(), itemgetter(0)), {'a':['aa','ab'], 'b':['bb']})

Here's an example of how to invert a grouping, using an int as key (which uses itemgetter; passing a str will use attrgetter), and using a val function:

d = {0: [1, 3, 7], 2: [3], 3: [5], 4: [8], 5: [4], 7: [5]}
groupby(((o,k) for k,v in d.items() for o in v), 0, 1)
{1: [0], 3: [0, 2], 7: [0], 5: [3, 7], 8: [4], 4: [5]}


last_index(x, o)

Finds the last index of occurence of x in o (returns -1 if no occurence)

test_eq(last_index(9, [1, 2, 9, 3, 4, 9, 10]), 5)
test_eq(last_index(6, [1, 2, 9, 3, 4, 9, 10]), -1)


filter_dict(d, func)

Filter a dict using func, applied to keys and values

letters = {o:chr(o) for o in range(65,73)}
{65: 'A', 66: 'B', 67: 'C', 68: 'D', 69: 'E', 70: 'F', 71: 'G', 72: 'H'}
filter_dict(letters, lambda k,v: k<67 or v in 'FG')
{65: 'A', 66: 'B', 70: 'F', 71: 'G'}


filter_keys(d, func)

Filter a dict using func, applied to keys

filter_keys(letters, lt(67))
{65: 'A', 66: 'B'}


filter_values(d, func)

Filter a dict using func, applied to values

filter_values(letters, in_('FG'))
{70: 'F', 71: 'G'}



Like itertools.cycle except creates list of Nones if o is empty

test_eq(itertools.islice(cycle([1,2,3]),5), [1,2,3,1,2])
test_eq(itertools.islice(cycle([]),3), [None]*3)
test_eq(itertools.islice(cycle(None),3), [None]*3)
test_eq(itertools.islice(cycle(1),3), [1,1,1])


zip_cycle(x, *args)

Like itertools.zip_longest but cycles through elements of all but first argument

test_eq(zip_cycle([1,2,3,4],list('abc')), [(1, 'a'), (2, 'b'), (3, 'c'), (4, 'a')])


sorted_ex(iterable, key=None, reverse=False)

Like sorted, but if key is str use attrgetter; if int use itemgetter



Create new function that negates result of f

def f(a): return a>0


argwhere(iterable, f, negate=False, **kwargs)

Like filter_ex, but return indices for matching items


filter_ex(iterable, f=noop, negate=False, gen=False, **kwargs)

Like filter, but passing kwargs to f, defaulting f to noop, and adding negate and gen


range_of(a, b=None, step=None)

All indices of collection a, if a is a collection, otherwise range

test_eq(range_of([1,1,1,1]), [0,1,2,3])
test_eq(range_of(4), [0,1,2,3])


renumerate(iterable, start=0)

Same as enumerate, but returns index as 2nd element instead of 1st

test_eq(renumerate('abc'), (('a',0),('b',1),('c',2)))


first(x, f=None, negate=False, **kwargs)

First element of x, optionally filtered by f, or None if missing

test_eq(first(['a', 'b', 'c', 'd', 'e']), 'a')
test_eq(first([False]), False)
test_eq(first([False], noop), None)


nested_attr(o, attr, default=None)

Same as getattr, but if attr includes a ., then looks inside nested objects

a = SimpleNamespace(b=(SimpleNamespace(c=1)))
test_eq(nested_attr(a, 'b.c'), getattr(getattr(a, 'b'), 'c'))
test_eq(nested_attr(a, 'b.d'), None)


nested_idx(coll, *idxs)

Index into nested collections, dicts, etc, with idxs

a = {'b':[1,{'c':2}]}
test_eq(nested_idx(a), a)
test_eq(nested_idx(a, 'b'), [1,{'c':2}])
test_eq(nested_idx(a, 'b', 1), {'c':2})
test_eq(nested_idx(a, 'b', 1, 'c'), 2)



Dict from value to index

test_eq(val2idx([1,2,3]), {3:2,1:0,2:1})


uniqueify(x, sort=False, bidir=False, start=None)

Unique elements in x, optionally sort-ed, optionally return reverse correspondence, optionally prepend with elements.

t = [1,1,0,5,0,3]
test_eq(uniqueify(t, sort=True),[0,1,3,5])
test_eq(uniqueify(t, start=[7,8,6]), [7,8,6,1,0,5,3])
v,o = uniqueify(t, bidir=True)
test_eq(o,{1:0, 0: 1, 5: 2, 3: 3})
v,o = uniqueify(t, sort=True, bidir=True)
test_eq(o,{0:0, 1: 1, 3: 2, 5: 3})


A tuple with extended functionality.

class fastuple[source]

fastuple(x=None, *rest) :: tuple

A tuple with elementwise ops and more friendly init behavior

Friendly init behavior

Common failure modes when trying to initialize a tuple in python:

> TypeError:'int' object is not iterable


tuple(3, 4)
> TypeError:tuple expected at most 1 arguments, got 2

However, fastuple allows you to define tuples like this and in the usual way:

test_eq(fastuple(3), (3,))
test_eq(fastuple(3,4), (3, 4))
test_eq(fastuple((3,4)), (3, 4))

Elementwise operations



+ is already defined in tuple for concat, so use add instead

test_eq(fastuple.add((1,1),(2,2)), (3,3))
test_eq_type(fastuple(1,1).add(2), fastuple(3,3))
test_eq(fastuple('1','2').add('2'), fastuple('12','22'))


* is already defined in tuple for replicating, so use mul instead

test_eq_type(fastuple(1,1).mul(2), fastuple(2,2))

Other Elementwise Operations

Additionally, the following elementwise operations are available:

  • le: less than
  • eq: equal
  • gt: greater than
  • min: minimum of
test_eq(fastuple(3,1).le(1), (False, True))
test_eq(fastuple(3,1).eq(1), (False, True))
test_eq(fastuple(3,1).gt(1), (True, False))
test_eq(fastuple(3,1).min(2), (2,1))

You can also do other elemntwise operations like negate a fastuple, or subtract two fastuples:

test_eq(-fastuple(1,2), (-1,-2))
test_eq(~fastuple(1,0,1), (False,True,False))

test_eq(fastuple(1,1)-fastuple(2,2), (-1,-1))
test_eq(type(fastuple(1)), fastuple)
test_eq_type(fastuple(1,2), fastuple(1,2))
test_ne(fastuple(1,2), fastuple(1,3))
test_eq(fastuple(), ())

Functions on Functions

Utilities for functional programming or for defining, modifying, or debugging functions.

class bind[source]

bind(func, *pargs, **pkwargs)

Same as partial, except you can use arg0 arg1 etc param placeholders

bind is the same as partial, but also allows you to reorder positional arguments using variable name(s) arg{i} where i refers to the zero-indexed positional argument. bind as implemented currently only supports reordering of up to the first 5 positional arguments.

Consider the function myfunc below, which has 3 positional arguments. These arguments can be referenced as arg0, arg1, and arg1, respectively.

def myfn(a,b,c,d=1,e=2): return(a,b,c,d,e)

In the below example we bind the positional arguments of myfn as follows:

  • The second input 14, referenced by arg1, is substituted for the first positional argument.
  • We supply a default value of 17 for the second positional argument.
  • The first input 19, referenced by arg0, is subsituted for the third positional argument.
test_eq(bind(myfn, arg1, 17, arg0, e=3)(19,14), (14,17,19,1,3))

In this next example:

  • We set the default value to 17 for the first positional argument.
  • The first input 19 refrenced by arg0, becomes the second positional argument.
  • The second input 14 becomes the third positional argument.
  • We override the default the value for named argument e to 3.
test_eq(bind(myfn, 17, arg0, e=3)(19,14), (17,19,14,1,3))

This is an example of using bind like partial and do not reorder any arguments:

test_eq(bind(myfn)(17,19,14), (17,19,14,1,2))

bind can also be used to change default values. In the below example, we use the first input 3 to override the default value of the named argument e, and supply default values for the first three positional arguments:

test_eq(bind(myfn, 17,19,14,e=arg0)(3), (17,19,14,1,3))


mapt(func, *iterables)

Tuplified map

t = [0,1,2,3]
test_eq(mapt(operator.neg, t), (0,-1,-2,-3))


map_ex(iterable, f, *args, gen=False, **kwargs)

Like map, but use bind, and supports str and indexing

test_eq(map_ex(t,operator.neg), [0,-1,-2,-3])

If f is a string then it is treated as a format string to create the mapping:

test_eq(map_ex(t, '#{}#'), ['#0#','#1#','#2#','#3#'])

If f is a dictionary (or anything supporting __getitem__) then it is indexed to create the mapping:

test_eq(map_ex(t, list('abcd')), list('abcd'))

You can also pass the same arg params that bind accepts:

def f(a=None,b=None): return b
test_eq(map_ex(t, f, b=arg0), range(4))


compose(*funcs, order=None)

Create a function that composes all functions in funcs, passing along remaining *args and **kwargs to all

f1 = lambda o,p=0: (o*2)+p
f2 = lambda o,p=1: (o+1)/p
test_eq(f2(f1(3)), compose(f1,f2)(3))
test_eq(f2(f1(3,p=3),p=3), compose(f1,f2)(3,p=3))
test_eq(f2(f1(3,  3),  3), compose(f1,f2)(3,  3))

f1.order = 1
test_eq(f1(f2(3)), compose(f1,f2, order="order")(3))


maps(*args, retain=noop)

Like map, except funcs are composed first

test_eq(maps([1]), [1])
test_eq(maps(operator.neg, [1,2]), [-1,-2])
test_eq(maps(operator.neg, operator.neg, [1,2]), [1,2])


partialler(f, *args, order=None, **kwargs)

Like functools.partial but also copies over docstring

def _f(x,a=1):
    "test func"
    return x-a

f = partialler(_f, 2)
test_eq(f.order, 1)
test_eq(f(3), -1)
f = partialler(_f, a=2, order=3)
test_eq(f.__doc__, "test func")
test_eq(f.order, 3)
test_eq(f(3), _f(3,2))
class partial0:
    "Like `partialler`, but args passed to callable are inserted at started, instead of at end"
    def __init__(self, f, *args, order=None, **kwargs):
        self.f,self.args,self.kwargs = f,args,kwargs
        self.order = ifnone(order, getattr(f,'order',None))
        self.__doc__ = f.__doc__

    def __call__(self, *args, **kwargs): return self.f(*args, *self.args, **kwargs, **self.kwargs)
f = partial0(_f, 2)
test_eq(f.order, 1)
test_eq(f(3), 1) # NB: different to `partialler` example



Instantiate t if it's a type, otherwise do nothing

test_eq_type(instantiate(int), 0)
test_eq_type(instantiate(1), 1)


using_attr(f, attr)

Change function f to operate on attr

t = Path('/a/b.txt')
f = using_attr(str.upper, 'name')
test_eq(f(t), 'B.TXT')

Self (with an uppercase S)

A Concise Way To Create Lambdas

This is a concise way to create lambdas that are calling methods on an object (note the capitalization!)

Self.sum()), for instance, is a shortcut for lambda o: o.sum().

f = Self.sum()
x = np.array([3.,1])
test_eq(f(x), 4.)

# This is equivalent to above
f = lambda o: o.sum()
x = np.array([3.,1])
test_eq(f(x), 4.)

f = Self.argmin()
arr = np.array([1,2,3,4,5])
test_eq(f(arr), arr.argmin())

f = Self.sum().is_integer()
x = np.array([3.,1])
test_eq(f(x), True)

f = Self.sum().real.is_integer()
x = np.array([3.,1])
test_eq(f(x), True)

f = Self.imag()
test_eq(f(3), 0)

f = Self[1]
test_eq(f(x), 1)

Self is also callable, which creates a function which calls any function passed to it, using the arguments passed to Self:

def f(a, b=3): return a+b+2
def g(a, b=3): return a*b
fg = Self(1,b=2)
list(map(fg, [f,g]))
[5, 2]




Copy a non-builtin function (NB copy.copy does not work for this)

Sometimes it may be desirable to make a copy of a function that doesn't point to the original object. When you use Python's built in copy.copy or copy.deepcopy to copy a function, you get a reference to the original object:

import copy as cp
def foo(): pass
a = cp.copy(foo)
b = cp.deepcopy(foo)

a.someattr = 'hello' # since a and b point at the same object, updating a will update b
test_eq(b.someattr, 'hello')

assert a is foo and b is foo

However, with copy_func, you can retrieve a copy of a function without a reference to the original object:

c = copy_func(foo) # c is an indpendent object
assert c is not foo
def g(x, *, y=3):
    return x+y
test_eq(copy_func(g)(4), 7)


patch_to(as_prop=False, cls_method=False)

Decorator: add f to cls

The @patch_to decorator allows you to monkey patch a function into a class as a method:

class _T3(int): pass  

def func1(self, a): return self+a

t = _T3(1) # we initilized `t` to a type int = 1
test_eq(t.func1(2), 3) # we add 2 to `t`, so 2 + 1 = 3

You can access instance properties in the usual way via self:

class _T4():
    def __init__(self, g): self.g = g
def greet(self, x): return self.g + x
t = _T4('hello ') # this sets self.g = 'helllo '
test_eq(t.greet('world'), 'hello world') #t.greet('world') will append 'world' to 'hello '

You can instead specify that the method should be a class method by setting cls_method=True:

class _T5(int): attr = 3 # attr is a class attribute we will access in a later method
@patch_to(_T5, cls_method=True)
def func(cls, x): return cls.attr + x # you can access class attributes in the normal way

test_eq(_T5.func(4), 7)

Additionally you can specify that the function you want to patch should be a class attribute with as_prop = False

@patch_to(_T5, as_prop=True)
def add_ten(self): return self + 10

t = _T5(4)
test_eq(t.add_ten, 14)

Instead of passing one class to the @patch_to decorator, you can pass multiple classes in a tuple to simulteanously patch more than one class with the same method:

class _T6(int): pass
class _T7(int): pass

def func_mult(self, a): return self*a

t = _T6(2)
test_eq(t.func_mult(4), 8)
t = _T7(2)
test_eq(t.func_mult(4), 8)


patch(f=None, as_prop=False, cls_method=False)

Decorator: add f to the first parameter's class (based on f's type annotations)

@patch is an alternative to @patch_to that allows you similarly monkey patch class(es) by using type annotations:

class _T8(int): pass  

def func(self:_T8, a): return self+a

t = _T8(1)  # we initilized `t` to a type int = 1
test_eq(t.func(3), 4) # we add 3 to `t`, so 3 + 1 = 4
test_eq(t.func.__qualname__, '_T8.func')

Similarly to patch_to, you can supply a tuple of classes instead of a single class in your type annotations to patch multiple classes:

class _T9(int): pass 

def func2(x:(_T8,_T9), a): return x*a # will patch both _T8 and _T9

t = _T8(2)
test_eq(t.func2(4), 8)
test_eq(t.func2.__qualname__, '_T8.func2')

t = _T9(2)
test_eq(t.func2(4), 8)
test_eq(t.func2.__qualname__, '_T9.func2')

Just like patch_to decorator you can use as_propas_prop and cls_method parameters with patch decorator:

def add_ten(self:_T5): return self + 10

t = _T5(4)
test_eq(t.add_ten, 14)
class _T5(int): attr = 3 # attr is a class attribute we will access in a later method
def func(cls:_T5, x): return cls.attr + x # you can access class attributes in the normal way

test_eq(_T5.func(4), 7)



Deprecated; use patch(as_prop=True) instead

Other Helpers


Enum = []

An Enum that can have its values imported

_T = ImportEnum('_T', {'foobar':1, 'goobar':2})
test_eq(foobar, _T.foobar)
test_eq(goobar, _T.goobar)


Enum = []

An ImportEnum that behaves like a str


str_enum(name, *vals)

Simplified creation of StrEnum types

_T = str_enum('_T', 'a', 'b')
test_eq(f'{_T.a}', 'a')
test_eq(_T.a, 'a')
test_eq(list(_T.__members__), ['a','b'])
print(_T.a, _T.a.upper())
a A

class Stateful[source]

Stateful(*args, **kwargs)

A base class/mixin for objects that should not serialize all their state

class _T(Stateful):
    def __init__(self):

t = _T()
t2 = pickle.loads(pickle.dumps(t))

Override _init_state to do any necessary setup steps that are required during __init__ or during deserialization (e.g. pickle.load). Here's an example of how Stateful simplifies the official Python example for Handling Stateful Objects.

class TextReader(Stateful):
    """Print and number lines in a text file."""
    def __init__(self, filename):
        self.filename,self.lineno = filename,0

    def readline(self):
        self.lineno += 1
        line = self.file.readline()
        if line: return f"{self.lineno}: {line.strip()}"

    def _init_state(self):
        self.file = open(self.filename)
        for _ in range(self.lineno): self.file.readline()
reader = TextReader("00_test.ipynb")

new_reader = pickle.loads(pickle.dumps(reader))
1: {
2: "cells": [
3: {

class PrettyString[source]

PrettyString() :: str

Little hack to get strings to show properly in Jupyter.

Allow strings with special characters to render properly in Jupyter. Without calling print() strings with special characters are displayed like so:

with_special_chars='a string\nwith\nnew\nlines and\ttabs'
'a string\nwith\nnew\nlines and\ttabs'

We can correct this with PrettyString:

a string
lines and	tabs


even_mults(start, stop, n)

Build log-stepped array from start to stop in n steps.

test_eq(even_mults(2,8,3), [2,4,8])
test_eq(even_mults(2,32,5), [2,4,8,16,32])
test_eq(even_mults(2,8,1), 8)



Get number of cpus



add_props(f, g=None, n=2)

Create properties passing each of range(n) to f

class _T(): a,b = add_props(lambda i,x:i*2)

t = _T()
class _T(): 
    def __init__(self, v): self.v=v
    def _set(i, self, v): self.v[i] = v
    a,b = add_props(lambda i,x: x.v[i], _set)

t = _T([0,2])
t.a = t.a+1
t.b = 3



Decorator to check param and return types at runtime

typed validates argument types at runtime. This is in contrast to MyPy which only offers static type checking.

For example, a TypeError will be raised if we try to pass an integer into the first argument of the below function:

def discount(price:int, pct:float): 
    return (1-pct) * price

with ExceptionExpected(TypeError): discount(100.0, .1)

We can also optionally allow multiple types by enumarating the types in a tuple as illustrated below:

def discount(price:(int,float), pct:float): 
    return (1-pct) * price

assert 90.0 == discount(100.0, .1)
def foo(a:int, b:str='a'): return a
test_eq(foo(1, '2'), 1)

with ExceptionExpected(TypeError): foo(1,2)

def foo()->str: return 1
with ExceptionExpected(TypeError): foo()

def foo()->str: return '1'
assert foo()

typed works with classes, too:

class Foo:
    def __init__(self, a:int, b: int, c:str): pass
    def test(cls, d:str): return d

with ExceptionExpected(TypeError): Foo(1, 2, 3) 
with ExceptionExpected(TypeError): Foo(1,2, 'a string').test(10)

Notebook functions



Same as get_ipython but returns False if not in IPython



Check if code is running in some kind of IPython environment



Check if the code is running in Google Colaboratory



Check if the code is running in a jupyter notebook



Check if the code is running in a jupyter notebook

These variables are available as booleans in fastcore.basics as IN_IPYTHON, IN_JUPYTER, IN_COLAB and IN_NOTEBOOK.

(True, True, False, True)