python使用globals中的变量，而没有在e-local范围中将其定义为global

Question

在以下脚本中：

def update_dict(key):
my_dict[key] = ...

mydict = dict()
k = ...
update_dict(k)

my_dict尚未在类中声明，但已在update_dict中使用
1-口译员允许这种情况发生。python不应该在这上面抛出一个错误吗
2-如果不在使用全局变量的本地范围内将其声明为全局变量，那么使用全局变量会带来什么问题？python是否根据变量名使用指向全局变量的指针？

---

这个片段是我的总结，如果你认为(也许是正确的？(我在总结中犯了错误，完整的代码会在这段后面。这里与mydict等价的是sites。

import sys
import csv
import argparse
import gzip
class SiteStats:
def __init__(self, g_size, g_seq):
self.num_reads = 0
self.called_sites = 0
self.called_sites_methylated = 0
self.group_size = g_size
self.sequence = g_seq
def update_call_stats(key, num_called_cpg_sites, is_methylated, sequence):
if key not in sites:
sites[key] = SiteStats(num_called_cpg_sites, sequence)
sites[key].num_reads += 1
sites[key].called_sites += num_called_cpg_sites
if is_methylated > 0:
sites[key].called_sites_methylated += num_called_cpg_sites
parser = argparse.ArgumentParser( description='Calculate methylation frequency at genomic CpG sites')
parser.add_argument('-c', '--call-threshold', type=float, required=False, default=2.0)
parser.add_argument('-s', '--split-groups', action='store_true')
args, input_files = parser.parse_known_args()
assert(args.call_threshold is not None)
sites = dict()
# iterate over input files and collect per-site stats
for f in input_files:
if f[-3:] == ".gz":
in_fh = gzip.open(f, 'rt')
else:
in_fh = open(f)
csv_reader = csv.DictReader(in_fh, delimiter='t')
for record in csv_reader:
num_sites = int(record['num_motifs'])
llr = float(record['log_lik_ratio'])
# Skip ambiguous call
if abs(llr) < args.call_threshold * num_sites:
continue
sequence = record['sequence']
is_methylated = llr > 0
# if this is a multi-cpg group and split_groups is set, break up these sites
if args.split_groups and num_sites > 1:
c = str(record['chromosome'])
s = int(record['start'])
e = int(record['end'])
# find the position of the first CG dinucleotide
sequence = record['sequence']
cg_pos = sequence.find("CG")
first_cg_pos = cg_pos
while cg_pos != -1:
key = (c, s + cg_pos - first_cg_pos, s + cg_pos - first_cg_pos)
update_call_stats(key, 1, is_methylated, "split-group")
cg_pos = sequence.find("CG", cg_pos + 1)
else:
key = (str(record['chromosome']), int(record['start']), int(record['end']))
update_call_stats(key, num_sites, is_methylated, sequence)
# header
print("t".join(["chromosome", "start", "end", "num_motifs_in_group", "called_sites", "called_sites_methylated", "methylated_frequency", "group_sequence"])
)
sorted_keys = sorted(list(sites.keys()), key = lambda x: x)
for key in sorted_keys:
if sites[key].called_sites > 0:
(c, s, e) = key
f = float(sites[key].called_sites_methylated) / sites[key].called_sites
print("%st%st%st%dt%dt%dt%.3ft%s" % (c, s, e, sites[key].group_size, sites[key].called_sites, sites[key].called_sites_methylated, f, sites[k
ey].sequence))

谢谢！

Answer 1

global varname只有在分配给全局变量时才需要，因为否则它将被假定为创建的新局部变量。对于所有其他情况，Python已经知道它应该是所使用的全局，并且没有歧义。

在您的示例代码中，您正在全局dict上设置一个键/值对，而不是分配给全局变量：

def update_dict(key):
my_dict[key] = ...

以下是需要global来修改全局b:的情况

b = 0
def a():
global b
b = 2
print(b)
a()
print(b)

输出：

0
2