[英]How to return a generator in Python
我正在考慮設計我的函數以結合數據庫查詢返回生成器。 但是對迭代器的概念有疑問
def func():
sql =" select some rows "
dbconn = "connect and open to dtabase code"
ret = ( execute(sql) ) <----- returning a generator?
dbclose <---- I close the db connection here, but it gives me error
return ret
問題是,當我在主函數中迭代生成器時,我碰到“關閉的游標錯誤”。 我應該在func()中關閉還是不關閉它? 我想當對func()的調用結束時,dbconn變量將超出范圍,我不必擔心關閉嗎?
# main function
for it in func():
do something with it
close dbconn here?
我應該如何設計? 重現像列表這樣的數據結構會更好嗎? 謝謝
您可以使用Context Manager ,例如(包含一些偽代碼):
from contextlib import contextmanager
@contextmanager
def func():
sql =" select some rows "
dbconn = "connect and open to dtabase code"
yield execute(sql) # returns this really a generator?
dbclose #pseudocode, you probably want to put this in a try/finally block
with func() as result:
for it in result:
do something with it
當然,這僅在execute(sql)
確實返回生成器時才有用。 如果在關閉連接之前將所有數據放入列表(進而放入內存),則問題將過時。
def func():
sql =" select some rows "
dbconn = "connect and open to dtabase code"
ret = list( execute(sql) )
dbclose # no problem here, since all data is already fetched
return ret
針對您的評論:
如果您的數據庫適配器遵循python DB API規范,則一種有效的方法是使用fetchmany
多次提取一堆行。
以下代碼以100個塊為單位獲取行,並在執行離開with
塊時顯式調用dbclose
:
def cursor_iter(cursor, num_of_rows=100):
while True:
rows = cursor.fetchmany(num_of_rows)
if not rows: break
for row in rows:
yield row
@contextmanager
def func():
sql = "select some rows"
dbconn = connect_and_open_database()
cursor = dbconn.cursor()
cursor.execute(sql)
yield cursor_iter(cursor)
dbclose()
with func() as result:
for row in result:
do_something(row)
我在使用數據庫方面經驗不足,但是我認為您應該檢索查詢結果並將其作為列表返回。 如果您確實需要一個迭代器(但我看不到為什么),則在列表ret
返回一個迭代器:
def func():
sql =" select some rows "
dbconn = "connect and open to dtabase code"
ret = execute(sql) # a list
dbclose()
return (elmt for elmt in ret) # return an iterator over ret
現在,如果它們存在一種檢索查詢的第n個元素的方法,例如execute(sql, n)
,如果n
太大則返回None
,則可以使用yield:
def func():
sql =" select some rows "
dbconn = "connect and open to dtabase code"
n = 0
ret = execute(sql,n) # return the n-th element
while ret is not None:
yield ret
n += 1
ret = execute(sql,n)
dbclose()
現在,這不是我所建議的,主要是因為在迭代器未完成時與數據庫的連接保持打開狀態。 如果某件事失敗或設計不當,它可能永遠不會發生。
關閉數據庫連接后,您將無法嘗試操作游標,我將嘗試使用這種方法:
def func(params):
sql = "query to execute"
cursor = execute(sql, params)
return cursor.fetchall() # retrieves all posible results as a sequence of sequences,
# i.g. list of tuples(*)
### Main ###
# Open database connection
# Create cursor
for elem in func(): # Call to retrieve desired element's method and do something with that
# Do something
# Close cursor
# Close database connection
(*) http://www.python.org/dev/peps/pep-0249/
希望對您有所幫助
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