Is there a pythonic way of knowing when the first and last loop in a for is being passed through?
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28-10-2019 - |
Вопрос
I have a template in which I placed, let's say 5 forms, but all disabled to be posted except for the first one. The next form can only be filled if I click a button that enables it first.
I'm looking for a way to implement a Django-like forloop.last templatetag variable in a for loop inside an acceptance test to decide whether to execute a method that enables the next form or not.
Basically what I need to do is something like:
for form_data in step.hashes:
# get and fill the current form with data in form_data
if not forloop.last:
# click the button that enables the next form
# submit all filled forms
Решение
If I am understanding your question correctly, you want a simple test for whether you are at the beginning or end of the list?
If that's the case, this would do it:
for item in list:
if item != list[-1]:
#Do stuff
For the first item in the list, you would replace "-1" with 0.
Другие советы
I don't know of anything built-in, but you can easily write a generator to give you the required information:
def firstlast(seq):
seq = iter(seq)
el = prev = next(seq)
is_first = True
for el in seq:
yield prev, is_first, False
is_first = False
prev = el
yield el, is_first, True
>>> list(firstlast(range(4)))
[(0, True, False), (1, False, False), (2, False, False), (3, False, True)]
>>> list(firstlast(range(0)))
[]
>>> list(firstlast(range(1)))
[(0, True, True)]
>>> list(firstlast(range(2)))
[(0, True, False), (1, False, True)]
>>> for count, is_first, is_last in firstlast(range(3)):
print(count, "first!" if is_first else "", "last!" if is_last else "")
0 first!
1
2 last!
You could use enumerate
and compare the counter with the length of the list:
for i, form_data in enumerate(step.hashes):
if i < len(step.hashes):
whatever()
for form_data in step.hashes[:-1]:
# get and fill the current form with data in form_data
for form_data in step.hashes[-1:]:
# get and fill the current form with data in form_data
# click the button that enables the next form
# submit all filled forms
Don't like the repetition of get and fill the current form with data in form_data
? Define a function.
Generator with buffer.
def first_last( iterable ):
i= iter(iterable)
f= next(i)
yield f, "first"
n= next(i)
for another in i:
yield n, None
n= another
yield n, "last"
for item, state in first_list( iterable ):
# state is "first", None or "last".
Zipping two sequences
flags = ["first"] + (len(iterable)-2)*[None] + ["last"]
for item, state in zip( iterable, flags ):
# state is "first", None or "last".
I think he wants to have a wrapper around the iterator that provides first / last queries, also the parameter could be an iterator so all sort of len() would faild
Here it is what I came up so far, the trick is to use a double iterator, one that looks ahead one step of the first:
class FirstLastIter(object):
def __init__(self, seq):
self._seq_iter = iter(seq)
self._seq_iter_next = iter(seq)
self._idx = -1
self._last = None
self.next_next()
@property
def first(self):
return self._idx == 0
@property
def last(self):
return self._last == True
def __iter__(self):
return self
def next_next(self):
try:
self._seq_iter_next.next()
except StopIteration:
self._last = True
def next(self):
val = self._seq_iter.next()
self._idx += 1
self.next_next()
return val
for x in FirstLastIter([]):
print x
iterator = FirstLastIter([1])
for x in iterator:
print x,iterator.first,iterator.last
iterator = FirstLastIter([1,2,3])
for x in iterator:
print x,iterator.first,iterator.last
returns:
1 True True
1 True False
2 False False
3 False True
Before anyone goes sharpening their torches or igniting the pitch-forks I'm no expert in what is Pythonic, that stated, seems to me that if first
and/or last
is wanted from a list, in the since of if first
or if last
within a loop, it seems expected to super
the class, and add-in the functionality that is desired... maybe, so what follows is totally pre-alpha version e-1^11% sorta code that may cause havoc if looked at on a Tuesday just the right way...
import sys
## Prevent `.pyc` (Python byte code) files from being generated
sys.dont_write_bytecode = True
from collections import OrderedDict
class MetaList(list):
"""
Generates list of metadata dictionaries for list types
## Useful resources
- [C Source for list](https://github.com/python/cpython/blob/master/Objects/listobject.c)
- [Supering `list` and `collections.MutableSequence`](https://stackoverflow.com/a/38446773/2632107)
"""
# List supering methods; maybe buggy but seem to work so far...
def __init__(self, iterable = [], **kwargs):
"""
> Could not find what built in `list()` calls the initialized lists during init... might just be `self`...
> If feeling cleverer check the C source. For now this class will keep a copy
## License [GNU_GPL-2](https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html)
Generates list of metadata dictionaries for lists types
Copyright (C) 2019 S0AndS0
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; version 2
of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
"""
self.metadata = []
for index, value in enumerate(iterable):
if isinstance(value, list):
sub_kwargs = {}
sub_kwargs.update(kwargs)
sub_kwargs['address'] = kwargs.get('address', [index])
sub_list = MetaList(iterable = value, **sub_kwargs)
self.append(sub_list, **kwargs)
else:
self.append(value, **kwargs)
# Note; supering order matters when using built in methods during init
super(MetaList, self).__init__(iterable)
def __add__(self, other):
"""
Called when adding one list to another, eg `MetaList([1,2,3]) + [9,8,7]`
- Returns copy of list plus `other`, sorta like `self.extend` but without mutation
## Example input
test_list = MetaList([1 ,2, 3])
longer_list = test_list + [4, 5, 6]
## Example output
print("#\ttest_list -> {0}".format(test_list))
# test_list -> [1, 2, 3]
print("#\tlonger_list -> {0}".format(longer_list))
# longer_list -> [1, 2, 3, 4, 5, 6]
"""
super(MetaList, self).__add__(other)
output = MetaList(self)
output.extend(other)
return output
def __setitem__(self, index, item, **kwargs):
"""
Called when setting values by index, eg `listing[0] = 'value'`, this updates `self` and `self.metadata`
"""
super(MetaList, self).__setitem__(index, item)
address = kwargs.get('address', []) + [index]
value = item
dictionary = self.__return_dictionary(
address = address,
index = index,
value = value)
self.metadata[index] = dictionary
self.__refresh_first()
self.__refresh_last()
self.__refresh_indexes(start = index)
def append(self, item, **kwargs):
"""
Appends to `self.metadata` an `OrderedDict` with the following keys
- `address`: `[0]` or `[0, 1, 5]` list of indexes mapping to `value`
- `index`: `0` or `42` integer of index within current listing
- `value`: `string`, `['list']`, `{'dict': 'val'}`, etc; not enabled by default
- `first`: `True`/`False` boolean; item is first in current listing
- `last`: `True`/`False` boolean; item is last in current listing
"""
super(MetaList, self).append(item)
# Update last status of previously last item within `self.metadata`
if self.metadata:
self.metadata[-1]['last'] = False
index = len(self.metadata)
address = kwargs.get('address', []) + [index]
value = item
dictionary = self.__return_dictionary(
address = address,
index = index,
value = value)
dictionary['first'] = False
dictionary['last'] = True
if len(self.metadata) == 0:
dictionary['first'] = True
self.metadata += [dictionary]
def extend(self, listing, **kwargs):
"""
Extends `self.metadata` with data built from passed `listing`
- Returns: `None`
> `kwargs` is passed to `MetaList` when transmuting list types
"""
super(MetaList, self).extend(listing)
for index, value in enumerate(listing):
if isinstance(value, list):
last_address = []
if self.metadata:
# Grab `address` list minus last item
last_address = self.metadata[-1]['address'][0:-1]
# Add this `index` to `address` list for recursing
sub_list = MetaList(value, address = last_address + [index], **kwargs)
self.append(sub_list, **kwargs)
else:
self.append(value, **kwargs)
def insert(self, index, item, **kwargs):
"""
Inserts `item` at `index` for `self` and dictionary into `self.metadata`
- Returns: `None`
Note: `self.metadata[index + 1]` have the following data mutated
- `data['index']`
- `data['address']`
Additionally: `self.metadata[0]` and `self.metadata[-1]` data mutations will occur
- `data['first']`
- `data['last']`
"""
super(MetaList, self).insert(index, item)
address = kwargs.get('address', []) + [index]
dictionary = self.__return_dictionary(
address = address,
index = index,
value = item,
**kwargs)
self.metadata.insert(index, dictionary)
self.__refresh_first()
self.__refresh_last()
self.__refresh_indexes(start = index)
# Off-set to avoid n +- 1 errors ;-)
self.__refresh_addresses(
start = index + 1,
index = len(address) - 1,
modifier = 1)
def pop(self, index = -1, target = None):
"""
Pop value from `self` and `self.metadata`, at `index`
- Returns: `self.pop(i)` or `self.metadata.pop(i)` depending on `target`
"""
popped_self = super(MetaList, self).pop(index)
popped_meta = self.__pop_metadata(index)
if 'metadata' in target.lower():
return popped_meta
return popped_self
def remove(self, value):
"""
Removes `value` from `self` and `self.metadata` lists
- Returns: `None`
- Raises: `ValueError` if value does not exsist within `self` or `self.metadata` lists
"""
super(MetaList, self).remove(value)
productive = False
for data in self.metadata:
if data['value'] == value:
productive = True
self.__pop_metadata(data['index'])
break
if not productive:
raise ValueError("value not found in MetaList.metadata values")
# Special herbs and spices for keeping the metadata fresh
def __pop_metadata(self, index = -1):
"""
Pops `index` from `self.metadata` listing, last item if no `index` was passed
- Returns: `<dictionary>`
- Raises: `IndexError` if `index` is outside of listed range
"""
popped_metadata = self.metadata.pop(index)
addr_index = len(popped_metadata['address']) - 1
## Update values within `self.metadata` dictionaries
self.__refresh_first()
self.__refresh_last()
self.__refresh_indexes(start = index)
self.__refresh_addresses(start = index, index = addr_index, modifier = -1)
return popped_metadata
def __return_dictionary(self, address, index, value, **kwargs):
"""
Returns dictionaries for use in `self.metadata` that contains;
- `address`: list of indexes leading to nested value, eg `[0, 4, 2]`
- `index`: integer of where value is stored in current listing
- `value`: Duck!... Note list types will be converted to `MetaList`
- `first`: boolean `False` by default
- `last`: boolean `False` by default
> `kwargs`: passes through to `MetaList` if transmuting a list `value`
"""
if isinstance(value, list):
kwargs['address'] = address
value = MetaList(value, **kwargs)
dictionary = OrderedDict()
dictionary['address'] = address
dictionary['index'] = index
dictionary['value'] = value
dictionary['first'] = False
dictionary['last'] = False
return dictionary
def __refresh_indexes(self, start = 0):
"""
Update indexes from `start` till the last
- Returns: `None`
"""
for i in range(start, len(self.metadata)):
self.metadata[i]['index'] = i
def __refresh_addresses(self, start = 0, end = None, index = 0, modifier = -1):
"""
Updates `address`es within `self.metadata` recursively
- Returns: `None`
- Raises: `TODO`
> `index` is the *depth* within `address` that `modifier` will be applied to
"""
if not start or start < 0:
start = 0
if not end or end > len(self.metadata):
end = len(self.metadata)
for i in range(start, end):
metadata = self.metadata[i]
if isinstance(metadata['value'], list):
metadata['value'].__refresh_addresses(index = index, modifier = modifier)
else:
if len(metadata['address']) - 1 >= index:
metadata['address'][index] += modifier
else:
raise Exception("# TODO: __refresh_addresses append or extend address list")
def __refresh_last(self, quick = True):
"""
Sets/re-sets `self.metadata` `last` value
- Returns `True`/`False` based on if `self.metadata` was touched
If `quick` is `False` all items in current listing will be touched
If `quick` is `True` only the last item and second to last items are touched
"""
if not self.metadata:
return False
if len(self.metadata) > 1:
self.metadata[-2]['last'] = False
if not quick and len(self.metadata) > 1:
for i in range(0, len(self.metadata) - 1):
self.metadata[i]['last'] = False
self.metadata[-1]['last'] = True
return True
def __refresh_first(self, quick = True):
"""
Sets first dictionary within `self.metadata` `first` key to `True`
- Returns `True`/`False` based on if `self.metadata` was touched
If `quick` is `False` all items will be touched in current listing
If `quick` is `True` the first and second items are updated
"""
if not self.metadata:
return False
if len(self.metadata) > 1:
self.metadata[1]['first'] = False
if not quick and len(self.metadata) > 1:
for i in range(1, len(self.metadata)):
self.metadata[i]['first'] = False
self.metadata[0]['first'] = True
return True
# Stuff to play with
def deep_get(self, indexes, iterable = None):
"""
Loops over `indexes` returning inner list or value from `self.metadata`
- `indexes` list of indexes, eg `[1, 3, 2]`
- `iterable` maybe list, if not provided `self.metadata` is searched
"""
referance = self.metadata
if iterable:
reference = iterable
for index in indexes:
reference = reference[index]
return reference
def copy_metadata(self):
"""
Returns copy of `self.metadata`
"""
return list(self.metadata)
def yield_metadata(self, iterable = None, skip = {'first': False, 'last': False, 'between': False}, **kwargs):
"""
Yields a *flat* representation of `self.metadata`,
Prefilter via `skip = {}` dictionary with the following data
- `first`: boolean, if `True` skips items that are first
- `last`: boolean, if `True` skips items that are last
- `between`: boolean, if `True` skips items that are not last or first
"""
metadata = self.metadata
if iterable:
metadata = MetaList(iterable).metadata
for item in metadata:
if isinstance(item.get('value'), list):
# Recurse thy self
for data in item['value'].yield_metadata(skip = skip, **kwargs):
yield data
else:
if skip:
if skip.get('first', False) and item['first']:
continue
if skip.get('last', False) and item['last']:
continue
if skip.get('between', False) and not item['first'] and not item['last']:
continue
# If not skipped get to yielding
yield item
... and it maybe buggier than the light fixtures of that one friend that talked to publicly of close encounters, they know who they are... but this does do some nifty tricks
Example input one
meta_list = MetaList([1, 2, 3, 4, 5])
for data in meta_list.metadata:
if data['first']:
continue
if data['last']:
continue
print("self[{0}] -> {1}".format(data['index'], data['value']))
Example output one
self[1] -> 2
self[2] -> 3
self[3] -> 4
Example input two
meta_list = MetaList(['item one', ['sub item one', ('sub', 'tuple'), [1, 2, 3], {'key': 'val'}], 'item two'])
for data in meta_list.yield_metadata():
address = "".join(["[{0}]".format(x) for x in data.get('address')])
value = data.get('value')
print("meta_list{0} -> {1} <- first: {2} | last: {3}".format(address, value, data['first'], data['last']))
Example output two
meta_list[0] -> item one <- first: True | last: False
meta_list[1][0] -> sub item one <- first: True | last: False
meta_list[1][1] -> ('sub', 'tuple') <- first: False | last: False
meta_list[1][2][0] -> 1 <- first: True | last: False
meta_list[1][2][1] -> 2 <- first: False | last: False
meta_list[1][2][2] -> 3 <- first: False | last: True
meta_list[1][3] -> {'key': 'val'} <- first: False | last: True
meta_list[2] -> item two <- first: False | last: True
If you feel your brains go all minty fresh, but it kinda isn't totally okay and that is in it's own way all the better... to me that be the most Pythonic
Enjoy, and perhaps if there be interest I'll push this to GitHub for all to Pull and Fork.
Side note @fabrizioM +1 for superb use of
@property
magics