I am trying to read a fasta file, manipulate is in Python (using BioPython) and then write it back.

The format of my sequences is like:

>k119_5 flag=0 multi=141.0706 len=473
AGGTTAGTCAGCACCGTTTCCGTGGTGCTGCCTTTCGCTTCAAAACCGACGGCGTCTATTACTGCATCCACGCCCCGGTGACCTGCCGTTTGTTCAATAATTGACTGTGCCGGATCGCTGTCTTCATCAAAATTAATCGGGATCGCGCCGTAGCGGTCGGCGGCGAAATGCAAGCGGTAGGGATGATGATCAACAACAAAAATCTGTTCCGCACCGAGCAACCGTGCACAGGCGATTGTCAACAATCCCACAGGACCAGCACCATAGACTGC
AACGCTTGAACCTTGTTGGATCTGCGCATTTTTTGCTGCCTGCCATGCCGTTGGCAGAATATCAGAAAGGAAAAGCGCTTTATCATCTGAAAGCAAAGGCGGTACTTTAAACGGCCCCACATTCCCTTTAGGGACGCGGACATATTCCGCCTGCCCACCAGGAACGCCGCCATACAGGTGACTATAACCAAACAATGCCGC


which is the result of the assmebly using Megahit. While trying to read, it only recognizes k119_5 as the id, while I want the whole first line as the id of the sequence. Is vital information I want to keep!

Any ideas how to work with this?

The description field in the SeqRecord object has the information you are looking for:
>> from Bio import SeqIO

Edit: as an aside, if you write a SeqRecord object using SeqIO's write method, the FASTA header should contain the whole original header.