## Package Description
[](https://dl.circleci.com/status-badge/redirect/gh/sfkleach/pushable/tree/main) [](https://pushable.readthedocs.io/en/latest/?badge=latest)
This is a Python package that provides a simple class Pushable that creates "pushable" iterators by wrapping an inner iterator/iterable. Pushable iterators act like dynamically expanding queues, allowing you to peek ahead or push items back onto the queue.
## Basic Usage
We can turn any iterable/iterator into a pushable iterator using the constructor.
```
count_up = Pushable( range( 0, 5 ) )
```
We can use it like an ordinary iterator:
```
print( next( count_up ) )
# Prints 0
```
Or we can look-ahead to see what is coming:
```
whats_up_next = count_up.peek()
print( whats_up_next )
# Print 1
print( next( count_up ) )
# Also prints 1 because peek does not remove the item from the internal queue.
```
We can even push back items onto it:
```
count_up.push("cat")
count_up.push("dog")
print( list( count_up ) )
# Prints 'dog', 'cat', 2, 3, 4
```
## Examples
From an iterator such as a file-object, which will iterate over the lines in a file, create a peekable/pushable iterator. This can be useful for example when we want to know if the iterator still has contents or want a sneak peek at what is coming.
```py
from pushable import Pushable
def read_upto_two_blank_lines( filename ):
with open( filename ) as file:
plines = Pushable( file )
# Pushable iterators can be used as booleans in the natural way.
while plines:
line = next( plines )
# peekOr makes it safe to look ahead.
if line == '\n' and plines.peekOr() == '\n':
# Two blank lines encountered.
break
else:
yield line
```
It is also useful to perform "macro-like" transformation.
```py
from pushable import Pushable
def translate( text, translations ):
ptokens = Pushable( text.split() )
while ptokens:
token = next(ptokens)
if token in translations:
ptokens.multiPush( *translations[token].split() )
else:
yield token
print( ' '.join( translate( 'My name is MYNAME', {'MYNAME':'Fred Bloggs'} ) ) )
# Prints: My name is Fred Bloggs
```
### More Complex Uses
In addition to peeking and popping items, which risks raising a
`StopIteration` exception if there's nothing left on the internal queue, we
can utilise `peekOr` and `popOr` to deliver a default value instead. The
default value is passed as an optional parameter and falls back to None.
We can also peek and pop multiple values using `multiPeekOr` and `multiPopOr`,
which return generators. These support skipping over values so that you can
get the 2nd and 3rd value without getting the first e.g.
```py
(second, third) = Pushable("pqr").multiPop(skip=1, count=2)
print( second, third )
# Prints: q r
```
Lastly, we can push multiple items with `multiPush`:
```py
count_up.multiPush("cat", "dog", "rabbit")
print( list( count_up ) )
# Prints: ['cat', 'dog', 'rabbit']
```
For a full set of available methods see [the documentation](https://pushable.readthedocs.io/en/latest/pushable.html).
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